Top Definition
When you see someone you dislike or have a problem with and you guys automatically break out into a scrap upon seeing one another.
1. "If that guy comes through its on sight"

2. "I saw that Evan guy at a party and it was on sight"
by AirMtb May 29, 2016

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A term in rock climbing, describes completing a lead route without falling. The climber must not know anything beforehand about the route or have watched someone else climb it. See flash and redpoint.
I made the first ascent of this route by onsighting it because it was probably only a 5.7.
by sano September 21, 2003

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