A Mushroom is when you dip your cock in paint or ink, And slap someone in the face with it when they are passed out. Thus leaving a mushroom imprint on their face.
"At this party last night, Kim was the first to pass out, So I gave her a mushroom"
The extreme version of Muffin Top. So extreme the top half grows from the head down then abruptly stops at the hips( like a mushroom ) to reveal thinner, and possibly even nice legs. Gravitational wise.. we don't know how this is possible. But many have proved gravity wrong by this odd shaped silhouette. Also known as Toad Stool.
"Wow, she has nice legs considering what her top half looks like."
"I know, she has a mushroom top, that's why she wears short skirts and then baggy sweatshirts."