Top Definition
A ninja boner is an erect penis that you must stealthily conceal whilst moving, which involves disguises and clever use of shadow and vegetation.

Often required when the words "Give it a minute" will not save you.
Last night Jen gave me such a ninja boner at the party, I had to creep around the outside of the garden in the dark to get a drink.
by Massive Jones May 20, 2007

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