An old retired sailor puts on his old uniform and heads for  the docks
once more for old times sake.

He engages a prostitute and takes her up to a room.

He's soon going at it as well as he can for a guy his age, but needing
some reassurance, he asks, "How am I doing?"

The prostitute replies, "Well old sailor, you're doing about  three
knots."
 
"Three knots?" he asks, "What's that supposed to mean?"

She says, "You're knot hard, you're knot in, and you're knot getting
your money back!"