Green’s Dictionary of Slang

F.O.W. phr.

(US) abbrev. indicating that a girl, taken out of town for a drive, has a simple choice if she wishes to return: fuck or walk.

Anecdota Americana 120: A fellow invited a girl friend into one of those F.O.W. cars, and when they were some way into the country, he asked her to prepare for a screw. She refused. He gave her the alternative to walk. fn. For the benefit of those not au courant [...] F.O.W. is the abbreviation for fuck or walk.