hobnail 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.

"side of fireplace," 1670s, alteration of hubbe (1510s), of unknown origin, perhaps somehow related to the first element in hobnail.

"solid center of a wheel," 1640s, of uncertain origin, perhaps, if all the senses are in fact the same word, from hubbe, originally probably "lump, round protuberance, boss," the source of the hob of a fireplace and the hobnail of a boot. A wheelwright's word, not generally known or used until c. 1828; it reached wider currency with the vogue for bicycling. Meaning "center of interest or activity or importance" first recorded 1858 in writings of Oliver W. Holmes, and originally especially of Boston.

Boston State-House is the hub of the solar system. [Holmes, "Autocrat of the Breakfast Table"]

[E]verybody knows that Boston used to be called the Hub, meaning the hub of the universe. It may still be the hub, because the center of a wheel moves slowly. [J.P. Marquand, Life magazine, March 24, 1941]