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1.
When a guy and a girl are going out and the girl just hangs all over the guy. The guy doesn't care. The girl is useless and hangs there much like a keychain to a backpack.
girl: (hugs boy)
boy: meh

observing girls: god, that girl is such a keychain
by TangoMaureen December 10, 2005
 
2.
The visible print made by a man's penis on the front of the pants. The phallic formation seen through a man's trousers, shorts, or jeans. See also DICK PRINT

Plural: keychains
We were too distracted by the keychain at the amusement park to really enjoy the roller coaster.
by q.wsmith March 31, 2010