The act of a serviceman or woman participating in sexual intercourse with a serviceperson of the opposite gender while stationed in the "field".
"Hey, Jim, did you field fuck Kim while y'all were OPFOR at the field training exercise the other day?"
A complely non-homosexual but very sexual looking act preformed mostly by US Marines and Army soldiers to piss off their comrades and or superiors. This generally involves one soldier being 'the bitch' and recieving less than welcome sexual acts to himself from the other marines.
Dude, you should have been there, we field fucked the shit out of Ted!