"pertaining to a nail or claw," 1834, from Latin unguis "a claw, nail of the finger or toe;" cognate with Greek onyx, Old English nægel, Old Norse nagl "nail;" see nail (n.).
Entries linking to ungual
nail n.
Old English negel "tapering metal pin," nægl "fingernail (handnægl), toenail," from Proto-Germanic *naglaz (source also of Old Norse nagl "fingernail," nagli "metal nail;" Old Saxon and Old High German nagel, Old Frisian neil, Middle Dutch naghel, Dutch nagel, German Nagel "fingernail; small metal spike"), from PIE root *(o)nogh "nail of the finger or toe" (source also of Greek onyx "claw, fingernail;" Latin unguis "fingernail, claw;" Old Church Slavonic noga "foot," noguti "fingernail, claw;" Lithuanian naga "hoof," nagutis "fingernail;" Old Irish ingen, Old Welsh eguin "fingernail, claw").
The "fingernail" sense seems to be the original one, but many figurative uses are from the "small metal spike" sense: hard as nails is from 1828. To hit the nail on the head "say or do just the right thing" is by 1520s; in Middle English driven in the nail (c. 1400) was "to drive home one's point, clinch an argument," and smiten the nail on the hed was "tell the exact truth" (mid-15c.). Phrase on the nail "on the spot, exactly" is from 1590s, of obscure origin; OED says it is not certain it belongs to this sense of nail.
As a unit of English cloth measure (about 2 1/4 inches) from late 14c.; perhaps from a nail being used to mark that length on the end of a yardstick.
ungulate adj.
"hoofed," 1802, from Late Latin ungulatus "hoofed," from ungula "hoof, claw, talon," diminutive (in form but not sense) of unguis "nail" (see ungual). Ungulata, the order of hoofed mammals, is recorded from 1839.