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3.
the act of beginning to fish, it can be fishing for fish, women or anything that you might be interested in catching at the time.
Ben "hey man you wanna go see that new movie?"

Joel "no man i can't i just got my fish-on im gonna be here all day"
by short round and dangerous April 21, 2009
 
1.
A term used to let others around you know their is a hot woman in the area. Only use this term if the woman is an 8 or above on a scale of 10. After yelling "Fish On" whoever sees this woman after you yelled it has to give confirmation by yelling, "Yea Buddy!". Women can help in search of a good catch. This works great for a work environment.
Jim: Fish On!
Everybody: Where at!
Dan: Yea Buddy!
by Bonez2127 January 08, 2012
 
2.
a subversive colloquialism, often used by male friends or 'mates', to comment on the fact that one of the males has captured the interest of some third party, most commonly a sexually viable female
Peter : Dude! Steph just added me on facebook.

John : Reckon she's keen?

Peter : Yeh dude, Fish On!
by jimmah32 August 12, 2014