CGU11, pronounced "see-gee-you-eleven", is a very old
US Navy prank played on hapless sailors who are not
salty enough to have heard of it.
Typically, the mark is told to stand out on the flight deck and to call in when the CGU11 is coming in for a landing. Usually they stay out there until they get bored. A good prankster would get an officer to go out every so often and tell the mark to keep their eyes peeled for any incoming CGU11's.
If you haven't been able to figure out what the prank is, the proper spelling of the incoming object is "seagull", or sea-GU-11.
The poor folks who fell for it were always the first ones to try to pull it on the other newbies. By that time, word got around and nobody else fell for it until a new batch of polywogs came on board.