Green’s Dictionary of Slang

showing off, acting in an arrogant or vulgar manner.

[1707		N. Ward Compleat and Humorous Account of Remarkable Clubs (1756) 63: Till they [i.e. one’s cheeks] were grown as unsizeable as the swanking Buttocks of a Wapping Hostess].
1914		Essendon Gazette (Vic.) 29 Jan. 1/3: Swanking in speech is impudence throwing its thin cloak over ignorance.
1950		A. Buckeridge Jennings Goes To School 172: There was meant to be subtle irony and reproach against swanking in Darbishire’s remark.