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1.
Adjective
1) A generic term for attempting to impress your peers.
2) Having feelings of sexual arousal whilst demonstrating your masculinity.
3) Can be used as a term for excessive consumption e.g bosing out on fairy cakes

Reference: Used by Arnold Schwarzenegger in the body building documentary "Pumping Iron".
Origins
The origins of the word have been hotly debated by academics and linguistic theorists worldwide. However, the most widely accepted explanations are: It is a combination of the words boss (a person who is (or believes s/he is) in change, and pose (to show-off); It is an adjective coined by bodybuilders in the late 70’s as a collective expression for the various poses they use on stage in ‘Mr. Universe’ style competition (e.g. “I was bosing out on stage in front of thousands of people”)

Modern usage
It has since been adopted by youth culture in the west where it is used as a prefix as well as a separate word (e.g. the scat-fest was bosetastic last night). This has led to the term transcending any singular or definable meaning and has instead become an attitude, behavior, comportment, appearance, style and Zeitgeist of post modern pop-culture.
by Sieb May 28, 2008