salutation in greeting, c. 1200, from Old Norse heill "health, prosperity, good luck," or a similar Scandinavian source, and in part from Old English shortening of wæs hæil "be healthy" (see health; and compare wassail).

The interj. hail is thus an abbreviated sentence expressing a wish, 'be whole,' i. e., be in good health, and equiv. to L. salve, plural salvete, or ave, plural avete .... [Century Dictionary]

"to greet or address with 'hail!,'" also "to drink toasts," c. 1200, heilen; to call to from a distance," 1560s (in this sense originally nautical), from hail (interj.). Related: Hailed; hailing. Bartlett ["Dictionary of Americanisms," 1848] identifies to hail from (1841) as "a phrase probably originating with seamen or boatmen." Hail fellow well met is from 1580s as a descriptive adjective, from a familiar greeting; hail fellow (adj.) "overly familiar" is from 1570s. Hail Mary (c. 1300) is the angelic salutation (Latin ave Maria) in Luke i.58, used as a devotional recitation. As a desperation play in U.S. football, attested by 1940. "Hail, Columbia," the popular patriotic song, also was a euphemism for "hell" in American English slang from c. 1850-1910.

Old English hær "hair, a hair," from Proto-Germanic *hēran (source also of Old Saxon, Old Norse, Old High German har, Old Frisian her, Dutch and German haar "hair"), perhaps from PIE *ghers- "to stand out, to bristle, rise to a point" (source also of Lithuanian šerys "bristle;" see horror).

Spelling influenced by Old Norse har and Old English haire "haircloth," from Old French haire, from Frankish *harja or some other Germanic source (see above). Hair-dye is from 1803. To let one's hair down "become familiar" is first recorded 1850. Homeopathic phrase hair of the dog (that bit you), remedy from the same thing that caused the malady, especially a drink on the morning after a debauch, 1540s in English, is in Pliny.