To have sex, usually out of extreme pity and/or drunkeness, or constant nagging, with a non erect penis. The action of trying to put it in the vagina is where dufflebag comes from.
Alcoholic- I got so drunk last night dude, I Duffledbaged Sarah because I felt sorry for her.
Friend- Ewwww. She is fucking ugly dude.
Alcoholic- That's why I only dufflebaged her.