"Make a higher offer for a house than (someone whose offer has already been accepted by the seller) and thus succeed in acquiring the property", 1920s (in sense 2): from Yiddish gezumph 'overcharge'. sense 1 dates from the 1970s.
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These days we associate gazumping with the buying and selling of houses, a use that dates from the 1970s. It now means ‘to raise the price of a house after accepting an offer from a prospective buyer’, but in the early 20th century it simply meant ‘to swindle’, deriving from Yiddish gezumph ‘to overcharge’. In the late 1980s the opposite term gazunder (a combination of gazump and under) was coined to describe the practice of lowering the amount of an offer that the seller has already accepted while threatening to withdraw if the new offer is not accepted.[gazump etymology, gazump origin, 英语词源]