Green’s Dictionary of Slang

mull n.2

[? SE mull, to grind, to pulverize or SE muddle]

1. a mess; usu. in phr. make a mull of, make a mess of.

[UK]‘An Amateur’ Real Life in London I 606: Somebody must make a mull* — but Randall’s the man — he is the favourite of the day, all the world to a penny roll. [* Mull — Defeat, loss, or disappointment].
[UK]Dickens Oliver Twist (1966) 350: Keeping out of the way arter that precious mull that you got me into, which might have cost me my neck.
[UK]Era (London) 3 June 3/4: I won’t go no further, in case I should make a mull on it (perhaps I have already).
[UK]F. Smedley Harry Coverdale’s Courtship 105: He’d made a regular mull of it.
T. Binney Light & Shadow Appendix No. viii 59: They really cannot themselves tell what the doctrine of the church actually is, although they speak of it as declared! The whole thing is a mull .
[UK]R.S. Surtees Facey Romford’s Hounds 186: He was very uncomfortable, and felt he was making a mull of it.
[UK]J. Hatton Clytie in Gentleman’s Mag. No. 235 720: And look what a mull you made of the old Earl business!
[UK]W.S. Gilbert ‘Phrenology’ Fifty ‘Bab’ Ballads 276: I must have made a mull, / This matter I’ve been blind in it.

2. (also mullah) a simpleton, a clumsy person; thus a country person.

[UK]Sam Sly 30 Dec. 4/1: [H]e has just taken from a ‘mull,’ surmounted by a real genuine cairngorm, a pinch of Irish blackguard, with which he is enthusiastically fraternising.
[UK]Partridge DSUE (8th edn) 765/2: from ca. 1865.
[Ire]P. Howard The Joy (2015) [ebook] I promised to beat the shite out of every mullah bastard that ever darkened the door of The Joy.

3. (Aus. drugs) marijuana, esp. when prepared (cleaned, sieved) for smoking.

[Aus]R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 44: The one holding the bowl dropped it and the mull went all over the floor.
[Aus]Tracks (Aus.) Dec. 23/4: They said if I didn’t smoke their two ounces of heavy-duty mull, man, then they’d kill ... er ... they’d kill you, Boss!
L. Jaivin Rock n Roll Babes from Outer Space 124: Tristram pinched some mull between his fingers and examined it closely.
[Aus]S. Maloney Something Fishy (2006) 106: Shit, pot, hemp, grass, weed, ganja, mull. Marijuana.
[Aus] A. Savage ‘Killing Peacocks’ in Crime Factory: Hard Labour [ebook] Adding under his breath that he grew the best mull in Gippsland.

In compounds

mullhead (n.)

(Aus. drugs) a regular smoker of marijuana.

[Aus]Tracks (Aus.) Feb. 3/3: I believe that the tide is turning and as this generation of surfers becomes tomorrow’s adults they will leave the drugs to the westies and the surfing ‘mull-heads’ will become a dinosaur.
Underground Surf Aust. 62/1: What sort of crew do you have up there? There’d be a lot of mullheads, wouldn’t there?
[Aus]R.G. Barrett Leaving Bondi (2013) [ebook] Warren, a mullhead working in an advertising agency.