Knock In Share definition Flag When someone knocks and then opens the door, defeating the whole purpose of knocking first. Guy: Dude, my mom tries to justify her barging in saying she knocked, but all she did was knock in. One of these days she's gonna catch me. Knock In by TheMagicalHobo September 28, 2008 0 0 Get merch Get the Knock In mug. Buy Now — $32.95 See more merch