Def. 1: Used to describe something particularly easy, generally a task.
Def.2: Used to describe someone of the opposite sex, particularly in how easy it will be to pick that person up. A sure thing.
origin: beginner's little league game of the same name, where the degree of difficulty in hitting a baseball is reduced by placing it on a tee for the child to hit.
"I know the guy who runs that place, so getting our friends in should be teeball tonight."