In golf, a tee is the small wooden support off of which you hit a golf ball. The word tee is also used more generally for the area of a golf course where you begin a match.
Other sports that use a tee include tee ball, American football, and rugby—each of these tees is significantly larger than a golf tee. As a verb, tee means "hit off of a tee," and in golf you "tee off" when play begins. Besides its Scottish origin, not much is known about where this word comes from. Informally, when you're teed off at someone, you're really angry.