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 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!