上当 literally means 'going to the pawnshop (当铺)' - historically, pawnshops often gave people far less than their items were worth, so going there meant getting a bad deal. Over time it came to mean being tricked in general.
usage
Often paired with 受骗: 上当受骗 = to be deceived and swindled. 吃亏上当 = to suffer losses from being tricked.
