c. 1600, "least pleasant, worst," in figurative phrase seamy side, from seam + -y (2); the seamy side of a sewn garment being the less attractive, and thus typically turned in. The popularity of the figurative sense likely is due to its use by Shakespeare in "Othello" IV.ii.146: "Some such Squire he was That turn'd your wits the seamy-side without, And made you to suspect me with the Moore."
