Why is the ninth month called September?
Old English negel "metal pin," nægl "fingernail (handnægl), toenail," from Proto-Germanic *naglaz (cf. 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" (cf. Greek onyx "claw, fingernail;" Latin unguis "nail, claw;" Old Church Slavonic noga "foot," noguti "nail, claw;" Lithuanian naga "hoof," nagutis "fingernail;" Old Irish ingen, Old Welsh eguin "nail, claw").
The "fingernail" sense seems to be the original one. Nail polish attested from 1891. To bite one's nails as a sign of anxiety is attested from 1570s. Nail-biting is from 1805. Hard as nails is from 1828. To hit the nail on the head "say or do just the right thing" is first recorded 1520s. Phrase on the nail "on the spot, exactly" is from 1590s, of obscure origin; OED says it is not even certain it belongs to this sense of nail.
Old English næglian "to fasten with nails," from Proto-Germanic *ganaglijanan (cf. 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. Meaning "to catch, seize" is first recorded 1766, probably from earlier sense "to keep fixed in a certain position" (1610s). Meaning "to succeed in hitting" is from 1886. To nail down "to fix down with nails" is from 1660s.
A fingernail or toenail.
A slender rod used in operations to fasten together the divided extremities of a broken bone.
A hypodermic needle (1960s+ Narcotics)verb
for fastening. (1.) Hebrew yathed, "piercing," a peg or nail of any material (Ezek. 15:3), more especially a tent-peg (Ex. 27:19; 35:18; 38:20), with one of which Jael (q.v.) pierced the temples of Sisera (Judg. 4:21, 22). This word is also used metaphorically (Zech. 10:4) for a prince or counsellor, just as "the battle-bow" represents a warrior. (2.) Masmer, a "point," the usual word for a nail. The words of the wise are compared to "nails fastened by the masters of assemblies" (Eccl. 12:11, A.V.). The Revised Version reads, "as nails well fastened are the words of the masters," etc. Others (as Plumptre) read, "as nails fastened are the masters of assemblies" (comp. Isa. 22:23; Ezra 9:8). David prepared nails for the temple (1 Chr. 22:3; 2 Chr. 3:9). The nails by which our Lord was fixed to the cross are mentioned (John 20:25; Col. 2:14). Nail of the finger (Heb. tsipporen, "scraping"). To "pare the nails" is in Deut. 21:12 (marg., "make," or "dress," or "suffer to grow") one of the signs of purification, separation from former heathenism (comp. Lev. 14:8; Num. 8:7). In Jer. 17:1 this word is rendered "point."