Top Definition
A swift, concise, piercing and high pitch "ahh", usually lasting only milliseconds, but the USN can be strung out to last a few seconds.
Sneaking into a flat mates room when he/she hasn't realised, getting as close to them as possible, and delivering a Universal Scare Noise (USN) to give them a fright.
by daddylongleach November 03, 2010

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