The verb form of first base (which means
French kissing or making out), "first basing" someone is kissing them passionately or making out with them. Used to great effect by
Strong Sad of
Homestar Runner fame when speaking to his brother Strong Bad, spied kissing a piece of paper.
Uh,
Strong Bad, were you just first basing it with that piece of
loose leaf?
I went to go ask my brother a question and caught him totally first basing his
study buddy.
I was so drunk last week that I ended up first basing my friend's girl.