Green’s Dictionary of Slang

1. alert, aware, au fait.

[1664		W. Killigrew Pandora Act I: Hold my Lord, you are out at first, one of us is very brown I assure you].
1834		‘I Am A Blowen Togg’d Out So Gay’ in Flare-Up Songster 16: I am sure there is none on the town / Who can do the thing that is right half so brown.
1835		‘The Frolicsome Methodist Parson’ in Knowing Chaunter 39: With toying he made her quite mute, / His vows lasted ten months, or nearly, / Till Miss Green found it was a BROWN suit, / And saw she was done very clearly.
1837		Marryat Snarleyyow I 4: He was in a brown study, yet looked blue.
1914		Joyce ‘The Dead’ Dubliners (1956) 197: ‘Well, I hope, Miss Morkan,’ said Mr Browne, ‘that I’m brown enough for you because, you know, I’m all Brown.’.

2. (Aus.) cunning, corrupt.

1911

Truth (Sydney) 1 Jan. 8/3: By and by they tumbles on it: / (My word, too, the game were brown).