To fullumber means to keep the passion alive in the bedroom if you are too tired to have sex when in a relationship. If you fullumber with someone you engage in the act to prepare yourself but never go any further.
When you fullumber with someone you are fullumbering.
Friend 1: What did you get up to last night?
Friend 2: ...We didn't have sex, we fullumbered.
Example 2: A couple comes out of a room, I bet they were fullumbering