In full "Not Now, We're on the Plane, You dumbass!" is a response to somebody saying or doing something prematurely, or when they aren't supposed to.
Can also be shortened to "Not Now, we're on the plane" or "We're on the plane, you dumbass!"
The phrase is derived from a line by Josh Duhamel's character in Transformers Revenge of the Fallen. His character was responding to another who had prematurely deployed his parachute while inside of a plane.
User A: So, to remove the virus follow Step 1, step 2, step 3, then step 4. But ignore step 3 if you are running on Windows Vista.
User B: So I finished Step 1 and 2, skipped 3 and went to 4.
User A: Are you using Vista?
User B: No.
User A: Well don't do step 4! We're on the plane, you dumbass!