Danny, an old retired sailor, puts on his old sailors 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 Danny, you're doing about three
knots."
 
"Three knots?" he asks, "What's the hell that supposed to mean?"

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