Green’s Dictionary of Slang

(orig. US) a speculator.

1896–9		(?) H. Lawson ‘A Gentleman Sharper and Steelman Sharper’ in Roderick (1972) 224: Why don’t he go to one of those big-bug boomsters he knows.
1901		W.A. Sun. Times (Perth) 7 July 1/1: A South Fremantle land shark is spinning a ‘Webb’ to catch the gullible groveller [...] the brazen boomster has evidently been studying the L.S.D. of loyalty.
1904		Sun. Times (Perth) 24 Apr. 4/7: George Gray, the gay and festive boomster.
1909		J. Ware Passing Eng. of the Victorian Era.