Comment by defrost

Colonial countries used it freely in the 1880s.

By the 1950s it was recognised as a slur

* 1955 Wall St. Jrnl. 30 Dec. 1/3 The Communist Party Boss: Again accused the West of colonialist aims.

* 1958 Manch. Guardian Weekly 12 June 8/3 Mr. Bandaranaike's Government uses methods which it would denounce as ‘colonialist’ if they were employed by others.

* 1959 ‘M. Derby’ Tigress ii. 85 ‘And you were ― ?’ ‘A colonialist, Madam. I exploited these unhappy natives to make a fortune for myself.‥ We colonialists thought only of private gain.’

examples taken from full O.E.D entry