Other forms: shortchanged; shortchanging; shortchanges
To shortchange is to give someone less than you owe them. If you hand over a five dollar bill for a one-dollar lemonade and get three dollars back, the lemonade seller shortchanged you.
The useful word shortchange is an American English coinage from the 1880s. It comes from the money sense of the noun change, and the idea that someone has deliberately withheld some of the change they owe you, leaving you short. You can use shortchange even when you're not talking about cash: "Our math teacher promised a pizza party on the last day of school, but she really shortchanged us! All we got was a bag of pretzels."