A sexual act. Upon completion of coitus, the male participant nuzzles his mouth between the woman's breasts, and does a zerbert. Mouth contact must be maintained on both breasts so that each one emits a zerbert sound of a different tone, much like that of the di-tonal singing buddhist monks.
Girl: What the fuck you doin to my titties?
Guy: Just a di-tonal monk tit zerbert. Ohm!