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1.
The act of biting down softly while giving head, so the teeth are gently scraping the wang. Not meant to shock the male, but just to startle him enough to make him pay attention and not start thinking of something else while he is sitting back.
Bro 1: "Yo brosef! I heard you got some last night from that girl at the bar! How was she?"

Bro 2: "Well it was going pretty good. She was suckin' on my thang and I was just sitting there thinking about lax and stuff, when she started cheese grating me out of nowhere. It was kind of weird, but pretty rad at the same time.
by Old Saggy Man May 19, 2008
 
2.
When someone bites or "grates" whilst giving a blow job.
Often causes much discomfort for the penis being grated
Past tense-cheesegrated
Boy: My cock is still red after you were cheesegrating me last night
Girl: don't worry, it wont stop it getting up
by Dinah Soares December 31, 2009