A sexual act involving anal sodomy, in which one partner penetrates the other without lubrication, then proceeds to push the gluteal muscles together. When done correctly, the resulting friction leads to diminished mobility, replicating a tugging feeling on one's member.
Guy 1: It's a shame your girlfriend sprained her wrists, i heard she gives great handjobs.
Guy 2: It's cool, she let me do a Mississippi Tugboat. My dick is still sore.
"Anal sex is great, but a Mississippi Tugboat really lets you experience the grip."