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1 definition by shag master

When a woman orgasms because something was so great but no build up or sexual action was needed. Causes for an insta-o could be a hot guy, delicious food, or something just completely awesome.
Girl 1: Did you see that guitar player?
Girl 2: Yea, we made eye contact and had an insta-o.
by shag master August 29, 2010