A large and extra stinky bowel movement that you break into a sweat taking. It is usually time consuming and you have to bite on a washcloth while taking it. This is fun for guys who have been drinking to joke about using a different sort of accent, usually proper British.
"Ryan was in the bathroom for a half an hour taking a grisly steamer."
"That grisly steamer almost had me calling 911."
"Shawn is still talking about a grisly steamer he took last year."
"If you want to repulse a guy and you're a girl, talk about taking a grisly steamer."
"That grisly steamer made me want to slap my mom."