Green’s Dictionary of Slang

(orig. milit.) shameless, impudent; thus brass-neck it, to be shameless or impudent.

1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 35: Brass Neck: Impudent, e.g., A brass neck lie.
1932		Aberdeen Jrnl 16 Dec. 8/4: The sketch portrayed the imposition of a ‘brass-necked’ Member of Parliament [...] on the hospitality of a young married couple.
1941		Sun. Post 28 Dec. 8/1: The brass-hat who isn’t brass-necked enough to demand the things his men have a right to isn’t worth his cross-swords on his shoulder.
1973		(con. mid-1960s) J. Patrick Glasgow Gang Observed 79: Wee Frankie, pure mental he is, brass-necked it, walkin’ up the road wi’ his stupid, mid-grey flannels. [Ibid.] 81: Is that no’ brass neckin’ it, askin fur a match?
1995		I. Rankin Let It Bleed 158: Not even Tresa McAnally was brass-necked enough to repeat the lie.
2004		(con. 1990s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 403: He [...] weathered the very public humiliation and castigation like the brass-necked bastard he was, by blaming the whole thing on the Director General of the prison service.