Green’s Dictionary of Slang

1. (also gazoomph, gezumph) to swindle; thus gazumper, a swindler.

1928		Western Dly Press 19 Dec. 9/3: The man who had gone to Blackpool and had been ‘gazoomphed’ was not anxious to advertise his gullibility.
1928		Aberdeen Jrnl 19 Dec. 3/7: ‘Gazoomphing a sarker’ meant robbing a man who had money .
1934		P. Allingham Cheapjack 189: Grafters speak a language comprised of every possible type of slang [...] Quite a number of words are Yiddish. These include ‘gezumph,’ which means to cheat or to overcharge. [Ibid.] 319: Gezumpher, a swindler.
1936		Eve. Teleg. (Dundee) 1 Aug. 7/7: A fine old sport of ‘gazoomphing’ (mock auctions) still flourishes; but the increasing wariness of the ‘pigeons,’ together with the increased vigilance of the police has made it a poor sport to follow.
1961		Daily Mail 25 Feb. 9/2: M.P.s had admitted that they had been ‘gazoomphed’ by fast-talking racketeers .
1961		T. Taylor Baron’s Court All Change (2011) 94: ‘I bag the straight punters for a caser, and the mugs I gazump for what I can get’.
2019		C. Hammer Silver [ebook] Speight gazumping St Clair on the cheese factory.

2. (orig. estate agent) to accept a stated price for one’s property and then to raise that price, using as a threat a supposed, but usu. non-existent, ‘offer’ from elsewhere; alternatively, the seller accepts one price and then, tempted by a genuinely greater offer, dumps the first buyer without sorrow or ceremony.

1972		Guardian 12 May 21: The rapid increase in prices, the growing number of people with mortgages in their pockets has meant that there is little time to be choosy before you are gazumped.
1996		K. Lette Mad Cows 77: Loopholes in the law mean that richer couples can gazump those going through the legal channels.

3. in ext. use of sense 2, to take over, to usurp (in non-housing contexts).

2002

S. Maloney Something Fishy (2006) 30: Long story short, got to gazump your sight-seeing trip.