Grade 5 thumbtack Get Printable Handwriting (thumb-tack) 2 syllables Learn It! Spelling Test Definition noun. a tack with a wide flat head that can be pressed into a board with the thumbverb. fasten with above tack Example Sentence I used a thumbtack to hold the paper to the message board. Synonyms tack; pushpin; drawing pin Antonyms bulletin board Share with your friends: