To get a boner when incredibly relaxed, for no apparent reason. Usually occurs just before you go to bed.

The temporal opposite of a morning glory
Man, I'm so sleepy. Oops there goes the dusk tusk. Time for bed.

Mum: Jonny, time for bed! What on Earth is that in your pants?

Dom: Oh, don't worry ma, it's just the dusk tusk, I'm real sleepy.
by Sorsedetrontrou Fugazi June 1, 2009
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