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.
Old English næglian "to fix or fasten (something) onto (something else) with nails," from Proto-Germanic *ganaglijan (source also of Old Saxon neglian, Old Norse negla, Old High German negilen, German nageln, Gothic ganagljan "to nail"), from the root of nail (n.). Related: Nailed; nailing. The colloquial meaning "secure, succeed in catching or getting hold of (someone or something)" is by 1760; hence "to arrest" (by 1930). Meaning "to succeed in hitting" is from 1886. To nail down "to fix down with nails" is from 1660s.