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When you put your middle finger up to somebody behind an opaque object (predominantly a door), releasing anger but not getting into trouble. Usually used by young teens, aimed at parents.
"My mom was being so annoying... so i sly signed her through the door when i left the room."

"No way dude!"
by Landoddage May 27, 2009

Words related to Sly Sign

sign signing sly slying slysign