Green’s Dictionary of Slang

1. to waste, i.e. luck or money.

1850

‘Epistle from Joe Muggins’s Dog’ in Era (London) 3 Nov. 5/3: Well, ther Squire's got a slice of luck at last, and sarve him rite, but I hopes he woant mull it az he did in Ratan’s yere.

2. (UK Und.) to spend money.

1859

Matsell Vocabulum.

3. to think about, to cogitate upon, to work over in one’s mind.

1873		J. Fiske in J.S. Clark Life John Fiske I 488: [He] hopes I will add the chapter on ‘Matter and Spirit’ which I have been mulling for a year back [DA].
1932		T.J. Grayson Leaders and Periods of Amer. Finance 462: On this occasion he was lying ill in bed in his house on 55th Street, and of course mulling over the big deal which meant so much to him and his friends [DA].
1942		N. Algren Never Come Morning (1988) 1: The Mex backpedaled, mulling his man.
2001		N. Barlay Hooky Gear 34: The rest is just rose specs. Me mullin, dreamin.

4. (Aus.) to make a fool oneself; to spoil.

1895

C. Crowe Aus. Sl. Dict. 51: Mull, to make a fool of one’s self; to spoil anything.

5. (Aus. drugs, also mull up) to chop and mix cannabis finely for consumption [but note mouli v.].

1985		Tracks (Aus.) Oct. 7: Method: Mull all herbs (dry mix). Sprinkle on preheated hash.
1986		R.G. Barrett Real Thing 126: He’d mulled up enough dope for about four little cones.
1994		Mary Jane Issue Zero Sept. 🌐 Smokin’ Slang [...] Mull: To chop up one’s cannabis for easier packing.
1997		J. Birmingham Tasmanian Babes Fiasco (1998) 215: I remember there’d been so much shit to get through that we’d stopped bothering to mull it up.
2007		D. McDonald Luck in the Greater West (2008) 227: I’d love a cone too. – Sure, babe. I just mulled up.