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1.
Exaiming anothers body, especially chest, crouch and butt regions. Usually people do this to stranger but occsionly to people they know. Usually done somewhat discreetly
The guy at the park was checking out the hot girl as she walked by
by Santas Sexy Helper July 28, 2004
 
2.
to commit suicide; to end your own life,

stems from checking out of a hotel, checking out of your own life, choosing to leave.
i fucking gone im gone im gone, so im checking out. im checking out real soon.
by john gold March 15, 2007
 
3.
a term used on instant messaging services. meant as "im gunna go"
urscreenamesuks: yo, im gunna check out, i'll catch ya lata
by morgan April 16, 2005