Slang, possibly from the end of World
War II, but it was certainly used by the US Army during the Vietnam
War. It means a heavy or
awkward load in adverse or difficult circumstances.
When they first heard it, the expression “humpin’ the
pig” the word humpin' caused a certain amount of consternation among reporters (see humping). The pig in
question was not a pink bacon producer or an officer of the law but nickname for the M60 machine gun, so the
grunt they were interviewing would have been a machine gunner.