A sexual foreplay act where a chick shoves a guys' face between her breasts and shakes it a little.
Oh yeah, we were marshmallowing last night and then things really got hot!
Making out with someone and they happen to bite your lips and tongue, constantly. The consistent chewing can lead to numb lips and tongues.
me: last night, oh the marshmallowing!
her: ohjeez, did he really bite you that much?!
me: lets just say, my lips are still numb.