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1.
While mooning someone, the mooner bends over exposing his dick and balls. He then sways back and fourth as to make his dick look like the long pendulum on a grandfather clock
It just started as a casual moon, but then turned into a grandfather clock
by wing ding June 01, 2006
 
2.
When one gets on a table without your pants on, and swing your junk left to right.
I did a Grandfather-Clock to your mom
by FM4k September 07, 2003
 
3.
a big hangy thing that ticks when it swings
A HUGE DICK (swings like a grandfather clock)
by UrbanAsassin69 June 03, 2010
 
4.
The grandfather clock is performed while suspended in the air, generally with the help of 2 others. It is when you are hovering above a sleeping persons face with your dick and/or balls hanging directly over their face, waking them with either a slap or a loud noise (preferably similar to a grandfather clock striking the hour).
Dude, Robb ba ba brain'd me, so I'm gonna get him back tonight with a grandfather clock.
by Azzypoo September 15, 2011
 
5.
When your girlfriend won't let you have sex with her because of her period.you then convince her to let you pull out her tampon with your teeth, you then sit on her chest and hold her arms down as you proceed to swing the blood soaked plug over her face like a pendulum
I gave Kristi a grandfather clock today
by brown Charlie August 08, 2008