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.
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"
They won't have a clue you're talking about them!! Until everyone's using this phrase of course.