Stiffi can be something cool, or just a slag for a boner.
1. My new shoes are so stiffi!

2. Bro that girl gives me such a stiffi!
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A double stiffy is when two guys get an erection at the same time from the same female.
Damn bro, that was crazy last night. I can't belive we got a double stiffy watching Jennifer Aniston bend over on the set of the TV show friend's.
by NaturallyCali March 1, 2016
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Getting an erection due to having to take a piss.

Similar to Morning Wood, except it happens while you're awake.
Ashley: Is that a gun in your pocket, or are you just happy to see me?
Cam: Neither; it's a piss stiffy.
by Camshot4 June 8, 2016
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Getting an erection due to having to take a piss.

Similar to Morning Wood, except it happens while you're awake.
Ashley: Is that a gun in your pocket, or are you just happy to see me?
Cam: Neither; it's a piss stiffy.
by Camshot4 June 8, 2016
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A brown stiffy is when a man preforms anal sex on a woman (or sir) and she poops whilst he is inside. This can be intentional or by accident.
Yeah man the other night I gave Rachel a brown stiffy.
by Ching_Chang_Chong December 17, 2015
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