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1.
When a large person lays out in the sun or uses a tanning bed to become tan.
My fat sister went to the beach without sunscreen and now she looks like a roasted marshmallow. The girls at the tanning salon were roasting a marshmallow all day in the winter.
by tsintao June 03, 2009
 
2.
When you know a girl is allergic to something and you put it in her vagina anyway.
I knew she was allergic to Big Red, but I stuck a piece of it in her and I was roasting a marshmallow.
by Marsmallow roaster November 28, 2006