Having a three some; having sex with 2 people at the same time. This could involve doing home base with the first person and doing third with the 2nd person at the same time. Finger one girl, fuck the other. or if 2 guys, having sex with one and anal with the other. (double home base) or girl is having sex with one and blow jobin' the other.
Last night at the bar, I took these twins back to my place and I hit sixth base with them doggy style
Like in the other terms of a relationship status, (bases 1-5), 'sixth base' is a way of saying that a relationship is at its highest and most intense point. The couple could be having extremely romantic and pleasurable intercourse, in which case one of them will state that they've experienced 'sixth base', like one says which base they've been to in other cases.
'How far have you been?'
'I've been to third base'
'How's it going with Lisa?'
'Awesome, last night we went to sixth base!'