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

When one gets on a table without your pants on, and swing your junk left to right.

The Grandfather Clock, also referred to as 'the Grandfather's Cock, is an intricate sexual act in which two men of roughly the same height suspend a woman of smaller stature upside down by her her ankles, swinging her back and forth onto each others erections like an erotic pendulum; the flesh hand of the world famous Grandfather Clock. With each 'swing of the clock', the woman of interest is penetrated from both the front and behind (commonly mistaken for a war on two fronts), alternating holes with each pleasurable sway. The partners in crime swinging the woman to and fro are each assigned a specific hole for their genitals to call home, depending on the directions of their erections, of course. The irony of this sexual act is that the participants will be so invested in reaching such a chronological level of ecstasy that their perception of time will fleet, and when they've finally finished, they'll realize that what felt like minutes of sensual fun was actually hours of tireless thrusting. The grandfather clock can make the best of the times out of the worst of times, but if executed improperly, it can make the best of times into an embarrassing trip to the emergency room (take it from me; I didn't know the penis could sprain until I fell victim to the grandfather clock's sensual wrath). While the clean up may be a disaster, it will have been well worth it afterwards; the benefits will come the second you do.

A sex position for a two male one female threesome in which the two men face each other and the woman is hung upside down via one knee over the shoulder of one man. She is then penetrated vaginally and anally by each man respectively and swung back and forth. Resembling the pendulum of a grandfather clock.

a big hangy thing that ticks when it swings

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

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).

When you are down to your “reserve” pair underwear or boxer shorts that have a hole in the bottom exposing your dick hanging down simulation a working grandfather clock.

A total upset. One of the biggest surprises you've seen in a long time. Like discovering Ma'am and George had a secret passage to upstairs for Webster, hidden away inside a faux grandfather clock.