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1 definition by Athena12

 
1.
A slang word for erection or hard-on. As an erection is the 'stiffing' of the penis, a 'stiffy' is an understandable slang word. Normally used by children first discovering sexual things.
Ethan and I were discussing sex and he got a stiffy from thinking about it.
by Athena12 March 26, 2009