I am working on a story set in WWII Britain, specifically involving the RAF. There is one character who flips off another. I have already done research into the proper offense gesture, but I have been unable to find whether the verb "to flip off" was in use at this time.
I have found the phrase on Wiktionary and in Merriam-Webster but neither location gives me a date. Likewise, any Google search ("flip off" origin, flip off wwii, flip off date) gives me any clear indicator.
Thus my question: would a WWII Brit used the phrase "to flip off"?