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from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A slim, pointed piece of metal hammered into material as a fastener.
• noun A fingernail or toenail.
• noun A claw or talon.
• noun Something resembling a nail in shape, sharpness, or use.
• noun A measure of length formerly used for cloth, equal to ¹/₁₆ yard (5.7 centimeters).
• transitive verb To fasten, join, or attach with or as if with a nail.
• transitive verb To cover, enclose, or shut by fastening with nails.
• transitive verb To keep fixed, motionless, or intent.
• transitive verb To stop and seize; catch.
• transitive verb To detect and expose.
• transitive verb To strike or bring down.
• transitive verb To perform successfully or have noteworthy success in.
• transitive verb Baseball To put out (a base runner).
• transitive verb Vulgar Slang To have sexual intercourse with.

from The Century Dictionary.

• noun A thin, flat, blunt layer of horn growing on the upper side of the end of a finger or toe.
• noun In entomology, the uncus.
• noun In ornithology, the hard horny end of the bill of any lamellirostral bird, as a duck or goose.
• noun The callosity on the inner side of a horse's leg near the knee or the hock.
• noun A pin or slender piece of metal used for driving through or into wood or other material for the purpose of holding separate pieces together, or left projecting that things may be hung on it.
• noun A stud or boss; a short metallic pin with a broad head serving for ornament.
• noun Same as shooting-needle.
• noun A unit of English cloth-measure, 2¼ inches, or of a yard. Abbreviated n.
• noun A weight of eight pounds: generally applied to articles of food.
• noun A straight stamping-tool.
• To fix or fasten with a nail or with nails; drive nails into for the purpose of fastening or securing: often with a preposition and an object, or with an adverb, to denote the result: as, to nail up a box; to nail a shelf to the wall; to nail down the hatches; to nail a joist into place; to nail it back.
• To stud with nails.
• Figuratively, to pin down and hold fast; make secure: as, to nail a bargain.
• To secure by a prompt action; catch.
• To make certain; attest; confirm; clinch.
• To trip up; detect and expose, as in an error.
• To spike (a cannon).
• Nautical, to spoil; frustrate the purpose of; make unlucky: as, to nail the trip (that is, spoil the voyage).

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) the horny scale of plate of epidermis at the end of the fingers and toes of man and many apes.
• noun The basal thickened portion of the anterior wings of certain hemiptera.
• noun The terminal horny plate on the beak of ducks, and other allied birds.
• noun A slender, pointed piece of metal, usually with a head{2}, used for fastening pieces of wood or other material together, by being driven into or through them.
• noun A measure of length, being two inches and a quarter, or the sixteenth of a yard.
• noun (Ordnance) a round projectile with an iron bolt protruding to prevent it from turning in the gun.
• noun iron in plates from which cut nails are made.
• noun in hand; on the spot; immediately; without delay or time of credit.
• noun to describe the most important factor.
• transitive verb To fasten with a nail or nails; to close up or secure by means of nails.
• transitive verb To stud or boss with nails, or as with nails.