Definition of "thumb-nail" [thumb-nail]

• The nail of the thumb. (noun)
• Hence, figuratively, something as small as the nail of the thumb; something quite small, as a sketch or a volume. (noun)
• As small as the nail of the thumb; hence, quite small: as, a thumb-nail sketch; a thumb-nail series of books.

The Century Dictionary (Public Domain)

• "When he discovered a blood-blister under a thumb-nail, which he had received the previous week, his self-identification became doubly sure, and he knew that those unfamiliar hands belonged to Marcus"
• "In particular, Cheles cites the "OK" sign made by Petrarch: "If the first finger touches the middle of the right-hand edge of the thumb-nail with its extremity, the other fingers being relaxed, we shall have a graceful gesture well suited to express approval or to accompany statements of facts, and to mark the distinction between our different points.""
• "On compact cameras, sensors are tiny, typically thumb-nail size or smaller."