Top Definition
The difference between the amount of money given to an over 21 year old person by an under 21 year old person to buy alcohol and the actual cost of the alcohol, kept by the over 21 year old person as a payment for the service.
Under-age person: Hey, this bottle cost $17 and I gave you $20 - where's my change?
Over-age person: Sorry... Under 21 Tax.
by jonukas May 04, 2007

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