priest of the blue bag, n. — Green’s Dictionary of Slang

priest of the blue bag n.

[the trad. colour of the bag in which they carry their gown and wig]

a barrister.

[1833		‘Tommarroo’ in Tommarroo Songster in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 339: He bore a blue bag in his claw, / To see as if his trade was law].
1849		C. Kingsley Alton Locke (1850) 180: He [...] showed himself as practised in every law quibble and practical cheat as if he had been a regularly ordained priest of the blue bag.