Green’s Dictionary of Slang

boldface adj.

brazen, impudent; used as a term of address.

[UK]H. Porter Two Angry Women of Abington G3: Well bold-face, but I meane to make yee stay.
[UK]B.E. Dict. Canting Crew.
[UK]Cibber Rival Fools I i: Why how now, Boldface! Is that your Manners to interrupt a Gentleman when he’s private.
[UK]‘Conny Keyber’ Apology for the Life of Mrs. Shamela Andrews 13: Why how now Saucy Chops, Boldface, says he.
[Scot]Hist. of Peter and Betteries frontispiece: A Fierce and Dangerous Combat with Betteries Boldface his Wife.

In phrases

play boldface (v.) (also play brassface)

(W.I./UK black) to pretend to be braver or more confident than one is; to put on a bold face to mask one’s guilt.

[WI]S. Selvon ‘Calypsonian’ in Bim V 17 in Selvon (1989) 146: The best way is to play brassface [...] and walk out cool. [Ibid.] 152: All you have to do is play boldface, and you could commit murder and get away free.
[WI]S. Selvon Lonely Londoners 29: It all well and good to play boldface in a small place like Trinidad, but when he landin London it would be different.