Online Etymology Dictionary

horse-chestnut (n.): 1590s, from horse (n.) + chestnut. A tree probably native to Asia, introduced in England c. 1550; the name also was extended to similar North American species such as the buckeye. Said to have been so called because it was food for horses, "but this is appar. a mere guess" [Century Dictionary] and the sense is perhaps "large," as in horseradish. The nut resembles that of the edible chestnut but is bitter to the taste.
horse-collar (n.): mid-15c., from horse (n.) + collar (n.).
horse-faced (adj.): "having a long, rough, ugly face," 1670s, from horse (n.) + face (n.).
horse-flesh (n.): also horseflesh, c. 1400, "horses collectively;" 1530s, "meat from a horse," from horse (n.) + flesh (n.). From 1520s as a color-name.
horse-marine (n.): 1824, "one of an imaginary corps of mounted sailors," hence "a person out of his element and unfit for his place" [Century Dictionary], from horse (n.) + marine (n.). However by 1878 the term was being used in fact in reference to cavalrymen pressed into marine service or seamen mounted as an improvised shore defense.
horse-meat (n.): c. 1400, "food for horses," from horse (n.) + meat (n.). From 1853 as "horse-flesh."
horse-play (n.): also horseplay, "rough, excessive play," 1580s, from horse (n.) with its associations of "strong, coarse" + play (n.).
horse-race (n.): also horserace, 1580s, from horse (n.) + race (n.1). Related: Horse-racing.
horse-shit (n.): also horse-shit, by 1935, from horse (n.) + shit (n.).
horse-whip (n.): also horsewhip, 1690s, from horse (n.) + whip (n.). As a verb, "to flog with a horse-whip," from 1768. Related: Horse-whipped; horse-whipping.
horseback (n.): "the back of a horse," especially the part upon which a rider sits, late 14c., from horse (n.) + back (n.). The alternative formerly was described in jest as footback [Century Dictionary].
horsefeathers (n.): "nonsense," 1927, said to have been coined by U.S. cartoonist Billy De Beck; perhaps a variant of horseshit "nonsense," though the latter is attested in print only from 1940s.
horsefly (n.): also horse-fly, type of insect extremely annoying to horses and cattle, late 14c., from horse (n.) + fly (n.).
horsehair (n.): late 14c., from horse (n.) + hair (n.). Specifically the hair of the mane and tail, used for making haircloth and stuffing cushions, etc.
horsehide (n.): also horse-hide, early 14c., from horse (n.) + hide (n.1).
horseless (adj.): 1670s, from horse (n.) + -less. Especially in reference to automobiles c. 1895-1910.
horseman (n.): c. 1200, from horse (n.) + man (n.).
horsemanship (n.): "equestrian skill, management of horses," 1560s, from horseman + -ship.
horsepower (n.): also horse-power, unit for measurement of the rate at which a motor works, 1805, from horse (n.) + power (n.); established by Watt as the power needed to lift 33,000 pounds one foot in one minute, which actually is about 1.5 times the power of a strong horse. Much abused in 19c. technical writing as "very fallacious," "shockingly unscientific," etc.
horseradish (n.): also horse-radish, 1590s, Cochlearia armoricia; the common name preserves the once-common figurative adjectival sense of horse as "strong, large, coarse," as in in obsolete horse mushroom (1866), horse-balm (1808), horse parsley, horse-mussel, Old English horsminte "horse mint." The "London Encyclopaedia" (1829) has horse emmet for a large kind of ant and horse marten "a kind of large bee." Also see radish.
Some nations have used the word bull as an augmentative; the English use the word horse, this being no doubt the largest animal of their acquaintance before the southern breeds of oxen were introduced. ["The Annual Review," London, 1804]
horseshoe (n.): also horse-shoe, late 14c. (early 13c. as a proper name), from horse (n.) + shoe (n.). Horseshoes as another name for the game of quoits is attested by 1822.
HORSE-SHOES, the game of coits, or quoits--because sometimes actually played with horse-shoes. [John Trotter Brockett, "A Glossary of North Country Words," 1829]
The belief that finding a horseshoe by chance is lucky is attested from late 14c., and the practice of nailing one above a doorway to prevent a witch entering (or leaving) was common in London down to c. 1800. Of a type of bend in a river, 1770, American English. The horse-shoe crab of the east coast of the U.S. so called by 1809, for its shape; earlier simply horse-shoe (1775); also horse-hoof (1690s), horse-foot (1630s), which Bartlett (1848) identifies as "the common name."
horsetail (n.): c. 1400, from horse (n.) + tail (n.). As a kind of plant, from 1530s.
horsewoman (n.): 1560s, "woman who rides on horseback," from horse (n.) + woman. Compare horseman. Related: Horsewomanship (1811).
horst (n.): 1893 in geology, from German Horst "mass, heap" (given its geological sense by Suess, 1883), from Old High German hurst "thicket," from Proto-Germanic *hursti-, from PIE *krsti- (source also of Middle Dutch horst "underwood," Old English hyrst "grove, wooded eminence"), from root *kert- "to turn, entwine" (see hurdle (n.)).
Horst Wessel: name of a Nazi activist and SA bandleader (1907-1930), author in 1929 of the lyrics to what became the German Nazi party anthem, known after as the Horst-Wessel-Lied ("Horst Wessel Song").
horsy (adj.): 1590s, from horse (n.) + -y (2). Related: Horsiness.
hortative (adj.): "encouraging, inciting," 1620s, from Latin hortativus "that serves for encouragement," from hortat-, past participle stem of hortari "to exhort, urge, incite" (see hortatory).
hortatory (adj.): 1580s, from Middle French hortatoire and directly from Late Latin hortatorius "encouraging, cheering," from hortatus, past participle of hortari "exhort, encourage, urge, incite, instigate," intensive of horiri "urge, incite, encourage."

This is conjectured to be from the reconstructed PIE root *gher- (5) "to like, want" (source also of Old English giernan "to strive, desire, yearn;" Gothic gairnei "desire;" Greek khresthai "to lack, want; use, make use of," kharis "grace, favor," khairein "to rejoice, delight in;" Sanskrit haryati "finds pleasure, likes," harsate "is aroused;" Avestan zara "effort, aim;" Russian zhariti "awake desire, charm").

Older in English is hortation (1530s), from Latin hortationem.
Hortense: fem. proper name, from Latin Hortensia, fem. of Hortensius, a Roman gens name, related to hortus "garden" (see yard (n.1)).
horticultural (adj.): "pertaining to the culture of gardens," 1768, from horticulture + -al (1).
horticulture (n.): 1670s, "cultivation of a garden," coined from Latin hortus "garden" (see yard (n.1)), probably on model of agriculture. Famously punned upon by Dorothy Parker.
horticulturist (n.): "gardener on a large scale," 1818, from horticulture + -ist. Earlier was horticultist (1754).
Horus: Egyptian hawk-headed god of dual relations, 1650s, from Latin Horus, from Greek Horos, from Egyptian Hor, said to mean literally "the high-flying one."
hosanna (interj.): Old English osanna, via Medieval Latin hosanna, Late Latin osanna, and Greek ossana, hosanna, from Hebrew hosha'na, probably a shortening of hoshi'ah-nna "save, we pray" (see Psalms cxviii:25), from imperative of y-sh- (compare yeshua "salvation, deliverance, welfare," for which see Joshua) + emphatic particle -na. Originally an appeal for deliverance; used in Christian Church as an ascription of praise, because when Jesus entered Jerusalem this was shouted by Galilean pilgrims in recognition of his messiahhood (Matt. xxi:9, 15, etc.).
hose (n.): late 13c., "covering of woven cloth or leather for the lower part of the leg, with or without feet," from late Old English hosa "covering for the leg," from Proto-Germanic *huson- (source also of Old Saxon, Old Norse hosa "covering for the leg between the knee and ankle," Middle High German hose "covering for the leg," German Hose "trousers," Danish hose "hose, stockings;" Middle Dutch hose, Dutch hoos "hose, stocking," also "spout, waterspout"), literally "covering," from PIE *(s)keu- "to cover, conceal" (see hide (n.1)). Old French hose, Old Spanish huesa, Italian uosa are of Germanic origin.

From mid-15c. as "close-fitting garment resembling tights worn by men and boys."
The hose of the middle ages generally covered the person from the waist to the toes; they were secured to the upper garment by points or some similar device. At times the covering of one leg and side of the body was of different material and color from that of the other side. In the sixteenth century the leg-coverings were divided into two parts, and the word hose was applied rather to the breeches, the covering of the lower part of the leg foot being called the stocking or nether-stock. [Century Dictionary]
Used in Middle English of various things resembling a stocking, such as the sheath or husk of an ear of grain; sense of "flexible rubber tube for conveying liquid" is first attested mid-14c.
hose (v.): c. 1300, "to furnish with stockings," from hose (n.). Meaning "to drench in water as from a hose" is from 1883. Related: Hosed; hosing.
Hosea: masc. proper name, from Hebrew Hoshea, literally "salvation," from stem y-sh- "to save."
hoser (n.): "contemptible person," also hose-head, by 1982, a term popularized by the Canadian parody comic sketch "Great White North" with the fictional McKenzie Brothers on SCTV.
hosier (n.): late 14c., hosyere "maker or seller of hose" (attested as a surname from late 12c.), from hose (n.) + -ier, French-influenced agent noun suffix. In 19c. the term often was applied to tailors who sold men's garments ready-made.
hosiery (n.): 1775, "stocking collectively, hose of all kinds," from hosier + -y (1). As "factory where hose is made," from 1803.
hospice (n.): 1818, "rest house for travelers," especially the houses of refuge and shelter kept by monks in the passes of the Alps, from French hospice "hospital, almshouse" (Old French ospice "hospice, shelter," also "hospitality," 13c.), from Latin hospitium "hospitable reception, entertainment; hospitality, bonds of hospitality, relationship of guest and host;" also "place of entertainment, lodging, inn, guest-house," from hospes (genitive hospitis) "guest; host," also "a stranger, foreigner" (see host (n.1)).

Sense of "home for the aged and terminally ill " is from 1879; hospice movement first attested 1978.
hospitable (adj.): "kind and cordial to strangers or guests," 1560s, from Middle French hospitable, which is formed as if from a Medieval Latin hospitabilis, from the stem of Latin hospitari "be a guest," from hospes (genitive hospitis) "guest" (see host (n.1)). The Latin adjective was hospitalis, but this became a noun in Old French and entered English as hospital. Related: Hospitably.
hospital (n.): mid-13c., "shelter for the needy," from Old French hospital, ospital "hostel, shelter, lodging" (Modern French hôpital), from Late Latin hospitale "guest-house, inn," noun use of neuter of Latin adjective hospitalis "of a guest or host" (as a noun, "a guest; the duties of hospitality"), from hospes (genitive hospitis) "guest; host;" see host (n.1).

The sense of "charitable institution to house and maintain the needy" in English is from early 15c.; meaning "institution for sick or wounded people" is first recorded 1540s. The same word, contracted, is hostel and hotel. The sense shift in Latin from duties to buildings might have been via the common term cubiculum hospitalis "guest-chamber." The Latin adjective use continued in Old French, where ospital also could mean "hospitable" and ospitalite could mean "hospital."
hospitality (n.): late 14c., "act of being hospitable," from Old French ospitalité "hospitality; hospital," from Latin hospitalitem (nominative hospitalitas) "friendliness to guests," from hospes (genitive hospitis) "guest; host" (see host (n.1)).
hospitalization (n.): 1873, noun of action from hospitalize.
hospitalize (v.): 1873, from hospital + -ize. "Freq[uently] commented on as an unhappy formation" [OED]. As verbs, hospitate is recorded from 1620s as "to lodge or entertain, receive with hospitality" but is rare; hospitize is from 1895. Related: hospitalized; hospitalizing.
hospitaller (n.): early 14c., from Old French ospitalier "one devoted to the care of the sick and needy in hospitals;" see hospital.
hospodar (n.): former title of appointed Ottoman governors of Moldavia and Wallachia, 1680s, from Old Church Slavonic gospodi "lord, master," literally "lord of strangers," from gosti "guest, friend" (see guest (n.)) PIE *ghostis- "stranger;" second element from PIE root *poti- "powerful, lord" (see potent).
hoss (n.): 1815, representing U.S. dialectal variant pronunciation of horse (n.), especially as applied to (large or coarse) persons.
host (n.1): "person who receives guests," especially for pay, late 13c., from Old French oste, hoste "guest, host, hostess, landlord" (12c., Modern French hôte), from Latin hospitem (nominative hospes) "guest, stranger, sojourner, visitor (hence also 'foreigner')," also "host; one bound by ties of hospitality."

This appears to be from PIE *ghos-pot-, a compound meaning "guest-master" (compare Old Church Slavonic gospodi "lord, master," literally "lord of strangers"), from the roots *ghosti- "stranger, guest, host" (source also of Old Church Slavonic gosti "guest, friend;" see guest (n.)) and *poti- "powerful; lord" (see potent). The etymological notion is of someone "with whom one has reciprocal duties of hospitality" [Watkins]:
The word ghos-ti- was thus the central expression of the guest-host relationship, a mutual exchange relationship highly important to ancient Indo-European society. A guest-friendship was a bond of trust between two people that was accompanied by ritualized gift-giving and created an obligation of mutual hospitality and friendship that, once established, could continue in perpetuity and be renewed years later by the same parties or their descendants. [Watkins, "American Heritage Dictionary of Indo-European Roots"]
The biological sense of "animal or plant having a parasite" is from 1857.