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.