Okay I found the definition myself.
gnarfle the garthak-To have sex with an exceedingly unattractive, usually fat person, especially whilst inebriated. Derived from the movie Coneheads wherein Beldar must fight a hideous, greasy tusk-beast.
Damn she was fugly, but my man was mackin her friend so I had to gnarfle the garthak
I believe your mommas obviously didn’t whip you enough when you were little. Ya’ll are bad. Just like my reprobate but lovable husband.
Guilty as charged, m'Lud. I blame society. ;-)