refers to an ugly chick, or any other unpleasant or tedious task. used colloquially, one could say "let's get grim," or "let's grim up." used in this sense, the whole phrase could be used in a situation where you have to fuck an ugly chick, otherwise known as "man up and do it, pussy." It could also be applied to doing any unpleasant or tedious task or simply getting serious about a task.
"i need to grim up and do this chick."
"let's grim up and finish this project."
"hahahaha, good joke. but let's get grim and do this."