Noun

1. The head of a pin. Frequently used in size comparisons.
   • 1810, Thomas Thomson, A System of Chemistry, Vol. 4, Bell & Bradfute, page 602,

     The moment the nitre was red hot, the coal, previously reduced to small pieces of the size of a pinhead, was projected in portions of one or two grains at a time…

2. An ignorant, naïve, foolish, and stupid person.
3. A telemark skier
4. A human head that is unusually tapered or small, often due to microcephaly, or a person with that trait. Often promoted in freak shows as "human pinheads".
   • 1939, Amram Scheinfeld and Morton David Schweitzer, You and Heredity, Frederick A. Stokes Co., page 155,

     The microcephalic idiot is an unfortunate with a "pinhead," sometimes exhibited as a "what's-it" in circus side-shows, whose mental age never goes beyond that of an imbecile.

   • 1943, Oliver Ramsay Pilat, Sodom by the Sea: An Affectionate History of Coney Island, Garden City Publishing, page 187,

     Zip the What-Is-It was simply a Negro idiot. … For half an hour at a time, David Belasco used to watch Zip at Coney Island. The producer insisted he saw signs of intelligence in the pinhead…

5. A newborn cricket used as food for pets.
   • 1994, Raymond E. Hunziker, Leopard Geckos, Publisher, ISBN 079380258X, page 16,

     A newly hatched gecko will need pretty small crickets, but you will not have to go all the way down to pinheads.

   • 2000, Manny Rubio, Scorpions: Everything About Purchase, Care, Feeding, and Housing, Barron's Educational Series, ISBN 0764112244, page 70,

     Crickets can be purchased in many sizes from newborns ("pinheads") to adults.