Green’s Dictionary of Slang

mull n.1

(Anglo-Ind.) a resident of the Madras Presidency.

[Ind]‘Quiz’ Grand Master 145: A well-known Mul. popp’d out his head. [footnote: An abbreviation for Mulkatany, a common appellation for Madras officers.] [OED].
[Ind]‘Sir Toby Rendrag’ Poems 26: [T]he European residents at Madras [have] the name of Mulls, from their liking of Mulligatawny soup.
[UK]New Sporting Mag. (London) Dec. 394: ‘Mull’ An ‘owdacious’ never-to-be satisfied denizen of Madras, a deadly rival to the ‘Qui-hyes,’ and always wanting to enter into competition with them on the turf, in the chase, or the jungle. NB — Very partial to beer.
[Ind] [Grant Colesworthey] Anglo-Indian Domestic Sketch 98: The various appellations of ‘Curry eaters’ – ‘Muls’– ‘Ducks, ‘ and ‘Qui hy’s’ by which the society of India has been honoured and distinguished.
[UK]C.W. ‘The Tiger-Slayers’ in Sporting Rev. Mar. 207: British officers of all grades and presidencies, whether Quihis, Mulls, or Ducks, can most thoroughly appreciate what is good in the eating line.
[Ind]G. Drago John Hobbs 3: ‘Qui hi, or ditcher;’ a ‘benighted Mull’, or an ‘ambitious Duck’. These are nick terms applied to the English residents at the three presidencies – Bengal, Madras, and Bombay respectively. [...] The Madressee is named a ‘Mull’ from his supposed fondness for mulligatawney soup.
[Ind]R.F. Burton Sind Revisited I 68: [A]nd lastly, ‘Mulls,’ or Madrassís from the Benighted Presidency, because they lived upon water and mulligatawny, or they made a ‘mull’ of everything they attempted.
[UK]H.A. Giles Subjects connected with the Far East (2 edn) 157: MULLS. Madras Englishmen are so called because of their good mulligatawny.