To do some bangin.. like rogerring your lady friend... slipping your todger up her... rat up a drainpipe etc. Fompen is the verb of "fomp".
Man: Hey Lady,
Lady: Yeah?
Man: Im in the mood for some fompen
Lady: Let's fomp!
Man: I wanna fomp you in the butt
Lady: Im in the mood for fompen too!