first base

plural first bases

1. countableuncountableball gamesThe base after home plate in a counter-clockwise path around a baseball infield.

   The runner took his lead off first base.

2. broadlycountablesingularsingular onlyuncountableCompletion of the first phase of an activity.
3. UScolloquialcountablesingularsingular onlyuncountableKissing, regarded as the first phase of a sexual relationship.

   I got to first base with Danny, and hope to get him naked this weekend.

   1 more example

   Every time I got to the part about her out with Stradlater in that damn Ed Banky's car, it almost drove me crazy. I knew she wouldn’t let him get to first base with her, but it drove me crazy anyway.

   1951 July 16, J[erome] D[avid] Salinger, The Catcher in the Rye, Boston, Mass.: Little, Brown and Company, →OCLC, page 104

4. countableuncountableblackjackThe betting spot located immediately to the left of the dealer, which is first to receive cards and first to act.