An old retired sailor puts on his old uniform and heads for the docks once more, for old times sake.
He engages a female person of accommodating morals* 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 person of accommodating morals 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 your knot getting your money back.
* a euphemism for prostitute lah!
And for those who do 'knot' know what a 'knot' is, the knot (pronounced not), used by sea and air navigators, is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph. Wikipedia