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1). A playful phrase spoken to friends as a term of endearment. The phrase "your a thumb" roughly means "your dumb" and is spoken to friends to emphasize their flaws, in a loving way.

2). The phrase "your a thumb" may also mean "you have a big forehead"
1) Katie: *falls*

Malia: "YOUR A THUMB"

2) Malia: "Brendon has such a big forehead"

Katie: "He's such a thumb"
by SodiumChloride11111 June 07, 2018
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