Bloom uses this phrase in the Sirens episode to explain Boylan’s success with women, most obviously his wife, who, impressed by his gold watch and expensive outfit, has probably surrendered to the brass in her lover’s face that very moment.1 None of the standard commentaries annotates this saying, which is documented in John Tobin’s comedy The Honey Moon, published a year after the author’s death, in 1805.
... skilful Physicians […] are shining examples, that a man will never want gold in his pocket, who carries plenty of brass in his face! [Act 3, scene 2, p. 48]
Tobin’s play was republished in Britain and America, and was a popular success, as this newspaper snippet quoting Tobin’s character Jaquez, shows:
Mr Muz remained only a season at Portmouth. He was a notorious character, remarkable alike for his impudence, vanity and deceit. He was a perfect Jeremy Diddler, and the success of his schemes strikingly exemplified the truth of Jaques’s shrewd remark that "a man never wanted any gold in his pocket as long as he carried plenty of brass in his face".
Era (1853), Sunday, 27 March
Joyce also quotes from Tobin in the Nausicaa episode at 13.301-2, as Vincent Deane suggested to Gifford.
An earlier source for the phrase is George Farquhar’s comedy Love and a Bottle, which premiered in 1698 (published 1699):
But an't I an impudent Dog? Had I as much Gold in my Breeches, as Brass in my Face, I durst attempt a whole Nunnery. [Act 3, p. 27]
Farquhar is preceded by the following source of 1660:
His Mask being now taken off, he begins to appear in his Colours; And truly I wonder, he that hath from his youth dealt so much in Silver and Gold, should now have so much Brasse in his Face, as so impudently and in Print to Patronize his mischiefs under the names of such persons as the Lord Mayor of London, the Lords Commissioners of the Great Seal, the Lord Chief Justice of England, [etc.].
The Great Trappaner of England Discovered, p. 4
The phrase seems to have been proverbial, based on "brazen-faced", which suggests hardened emotions and which has been documented since 1571. A later example extends the metaphorical use of metals to describe human qualities:
With four metallic qualifications a man may feel pretty certain of worldly success: They are gold in his pocket, silver in his tongue, brass in his face, and iron in his heart.
Illustrated Police News (1886), Saturday, 15 May