Online Etymology Dictionary

under-achiever (n.): also underachiever, 1953, from under + agent noun from achieve (v.). Under-achievement is recorded from 1951.
under-employed (adj.): 1908, "not used to optimum capacity," originally in reference to working persons, from under + past participle of employ (v.).
under-employment (n.): also underemployment, 1909, from under + employment.
underage (adj.): also under-age, 1590s, from under + age (n.).
underarm (adj.): 1816, "underhand" (in reference to a style of throwing), from under + arm (n.1). First attested 1908 in dressmaking sense of "seams on the lower half of the arm-hole;" as a euphemism for armpit, it is attested from 1930s, popularized by advertisers.
underbelly (n.): c. 1600, from under + belly (n.). In figurative sense of "most vulnerable part" it is recorded from Churchill's 1942 speech. Sometimes used erroneously or euphemistically in sense of "seamy or sordid part" of anything.
underbid (v.): 1610s, from under + bid (v.). Related: Underbidding.
underbred (adj.): "of inferior breeding, vulgar," 1640s, from under + past participle of breed (v.). Of animals, "not pure bred," attested from 1890.
underbrush (n.): "shrub and small trees in a forest," 1775, from under + brush (n.2). Originally American English; compare undergrowth, attested in the same sense from 1600.
undercarriage (n.): 1794, from under + carriage (n.). Meaning "landing gear of an aircraft" is recorded from 1911.
undercharge (v.): 1630s, from under + charge (v.).
underclass (n.): "subordinate social class," 1894, from under (adj.) + class (n.). A loan-translation of Swedish underklass.
underclassman (n.): "sophomore or freshman," 1869, American English, from under (adj.) + class (n.) in the school form sense + man (n.).
undercover (adj.): 1854, "sheltered," from under + cover (n.). Sense of "operating secretly" attested from 1920.
undercroft (n.): "crypt of a church; underground vault," late 14c., from under + croft.
undercurrent (n.): 1660s, "stream of water or air flowing beneath the surface or beneath another current," a hybrid formed from under + current (n.). The figurative sense of "suppressed or underlying character" is attested from 1817.
undercut (v.): late 14c., "to cut down or off," from under + cut (v.). In the commercial sense of "sell at lower prices" (or work at lower wages) it is first attested 1884. Figurative sense of "render unstable, undermine" is recorded from 1955, from earlier literal meaning "cut so as to leave the upper portion larger than the lower" (1874).
underdeveloped (adj.): 1892, in photography, from under + past participle of develop (v.). In reference to countries or regions, recorded from 1949.
underdog (n.): "the beaten dog in a fight," 1887, from under + dog (n.). Compare top dog "dominant person in a situation or hierarchy" (see top (adj.)).
I'm a poor underdog
But tonight I will bark
With the great Overdog
That romps through the dark.

[from "Canis Major," Robert Frost, 1928]
underdone (adj.): 1680s, in reference to cooked meat, from under + done. Old English underdon (v.), Middle English underdo meant "to put under, to subject, subjugate."
underdressed (adj.): also under-dressed, "too plainly dressed," 1759, from under (adv.) + past participle of dress (v.).
underestimate (v.): 1812, "to estimate at too low an amount," from under + estimate (v.). Meaning "to rank too low, undervalue" is recorded from 1850. Related: Underestimated; underestimating.
underexposed (adj.): 1861, in photography, from under + past participle of expose (v.).
underfeed (v.): 1650s, from under + feed (v.). Related: Underfed; underfeeding.
underfoot (adv.): c. 1200, underfot "under the feet," from under + foot (n.). Compare similarly formed Middle Dutch ondervoete. As an adjective, attested from 1590s; in reference to persons, "continually in the way," it is recorded from 1891. Middle English under fot meant "vanquished, overcome."
undergarment (n.): 1520s, from under + garment (n.).
undergird (v.): 1520s, from under + gird (v.). Related: Undergirded; undergirding.
undergo (v.): Old English undergan "obtain, get; undertake," from under + gan (see go (v.)). Compare similarly formed Middle Dutch ondergaen, Old High German untarkun, German untergehen, Danish undergaa. Sense of "submit to, endure" is attested from c. 1300. Meaning "to pass through" (an alteration, etc.) is attested from 1630s. Related: Undergone; underwent.
undergrad (n.): short for undergraduate, 1827.
undergraduate (n.): 1620s, a hybrid formed from under + graduate (n.). British used fem. form undergraduette in 1920s-30s. As an adjective, in the school sense, from 1680s.
underground (adv.): 1570s, "below the surface," from under + ground (n.). As an adjective, attested from c. 1600; figurative sense of "hidden, secret" is attested from 1630s; adjectival meaning "subculture" is from 1953, from adjectival use in reference to World War II resistance movements against German occupation, on analogy of the dominant culture and the Nazis. Noun sense of "underground railway" is from 1887 (shortened from phrase underground railway, itself attested from 1834).
Underground Railroad (n.): "network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.
undergrowth (n.): "shrubs or small trees growing amid larger ones," c. 1600, from under + growth.
underhand (adv.): mid-14c., "by secret means, stealthily, in a surreptitious manner," from under + hand (n.). Perhaps the notion is of the hand turned over (thus concealing what it holds). Compare Middle Dutch onderhanden "by degrees, slowly," Dutch onderhandsch "secret, private." The adjective is attested from 1540s. Old English under hand meant "in subjection, in (one's) control or power."
underhanded (adj.): in reference to a throw, etc., "performed or done with the knuckles turned under," 1807, from under + hand (n.). Compare underhand. As "in secret," from 1825; as "with too few people," from 1834. Related: Underhandedly; underhandedness.
underlay (v.): Old English under lecgan "to support by placing something beneath;" see under + lay (v.). Related: Underlaid; underlaying. Compare similarly formed Old High German Related: untarleccan, German unterlegen.
underlie (v.): Old English under licgan "to be subordinate to, to submit to;" see under + lie (v.2). Meaning "to lie under or beneath" is attested from c. 1600; figurative sense of "to be the basis of" is attested from 1852 (implied in underlying). Similar formation in Old High German untarliggan; German unterliegen.
underline (v.): 1721, "to mark underneath or below with a line," from under + line (v.). Similar formation in Dutch onderlijnen. Related: Underlined; underlining. The noun is attested from 1888.
underling (n.): late Old English, "one who owes allegiance to a sovereign or ruler," from under + diminutive suffix -ling.
underlying (adj.): 1610s, present participle adjective from underlie.
undermine (v.): c. 1300, undermyne, "render unstable by digging at the foundation," from under + mine (v.). The figurative sense "injure by invisible, secret, or dishonorable means" is attested from early 15c. Similar formation in Dutch ondermijnen, Danish underminere, German unterminiren. The Old English verb was underdelfan. Related: Undermined; undermining.
undern (n.): an obsolete Old English and Middle English word for "morning;" in Old English originally "third hour of the day; 9 a.m." (corresponding to tierce). Hence underngeweorc, undernmete "breakfast." Common Germanic: Old Frisian unden, Old Saxon undorn, Middle Dutch onderen, Old High German untarn, Old Norse undorn; of uncertain origin. By extension, "period from 9 a.m. to noon;" but from 13c. shifting to "midday, noon" (as in undern-mete "lunch," 14c.); and by late 15c. to "late afternoon or early evening."
underneath (adv.): Old English underneoðan, from under + neoðan "below" (see beneath).
undernourished (adj.): also under-nourished, 1820, from under + past participle of nourish (v.).
underpants (n.): 1931, from under + pants. Drove out drawers, knickers in this sense.
underpass (n.): 1904, American English, from under + pass (n.).
underpay (v.): 1817, from under + pay (v.). Related: underpaid (1762); underpaying.
underpin (v.): "support or prop," 1520s (figurative); 1530s (literal), from under + pin (v.). Related: Underpinned; underpinning.
underpinning (n.): late 15c., "action of supporting or strengthening from beneath," from under + present participle of pin (v.). Figurative sense of "prop, support" is recorded from 1580s.
underprivileged (adj.): 1896, from under + past participle of privilege (v.). Noun use (short for underprivileged persons) is attested from 1935.