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1.
See vertical smile also.

Vertical grimace is an adaptation on "vertical smile", referring to the female genitalia. A grimace, is a more horizontal facial expression than a smile, therefore assimilating more closely to a vagina shape when rotated 45 degrees; rather than a smile.
Scene..
4 LADS walking down the street, they spot a cluster of jailbait across the road.

"Look at the vertical grimace"
"It's very GRIM out here today"
"Grimace, 4o'clock"
"GRIMACE!!"

They won't have a clue you're talking about them!! Until everyone's using this phrase of course.
by NewquayJay June 01, 2011