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A slow fat one is what happens to a male inmate when he drinks to much hooch. Regardless of the actual size of the penis in question, the excessive hooch makes it difficult to FAP or to perform anal copulation.

Slow to climax. Fat because the penis will not stay erect, hence the fat/floppy semi erect penis.
I tried to do a little uphill gardening with my prison bitch but my "slow fat one" made it impossible.
by Thomas Bombadil January 30, 2011