A Smart-Arse-Tax is a form of punishment imposed on smart-arses by taking money/ other items of value from friends, peers etc when they demonstrate their ability to be a smart-arse.
Calling someone a smart arse is intended to pull them down a peg or two for adopting a position of superiority, especially intellectual:

(1) through sarcasm,

(2) by using obscure knowledge to score points off other people, or

(3) just being too f**king smug when they're right about something.

Imposing a Smart-Arse-Tax, actually fines them for this behaviour, giving the recipient a sense of achievement and control.
by A smart arse November 30, 2011
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