third base
/ˌθəːd ˈbeɪs/noun
1 Baseball. The third of the four bases which the batter must touch in succession in order to score a run.
2 Slang (chiefly North American ). The third of four notional stages of sexual activity, usually characterized as the stimulation of the genitals or (in later use) oral sex.
Origin
Mid 19th century; earliest use found in By-laws & Rules Knickerbocker Base Ball Club. From third + base.