To give someone dirty looks or to stare them down. Also known as "mad doggin".

Damn homie, that fool was grimmin. Let's punk his ass.
by Baby Blaze420 July 19, 2006
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(grimm-in)verb: To threatin to kill; to grim
ima b grimmin u if u don't shaddup!
by Elz3r September 27, 2005
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