Green’s Dictionary of Slang

1. repellent, filthy [muck n.¹ ].

1900		Sporting Times 27 Jan. 3/4: ’Oo the ——ee? One o’ them muckin’ Germans?
1906		E. Pugh Spoilers 113: ‘’Ere, come on, Snuff,’ said Deuce, eyeing Mr. Gandy fiercely. ‘We’ll get out o’ this, quick. They can keep their muckin’ charity for them as ain’t quite so particular as us.’.

2. a euph. for fucking adj.

1917		B.E.F. Times 1 Nov. (2006) 226/2: What’s that, serg’int? – Give them muckin’ Fritzes, ’ell?
1938		L. Ortzen Down Donkey Row 174: I’d fallen be’ind wiv the muckin’ instalments.
1942		A. Gwynn-Browne F.S.P. 49: Eric he said this mucking bastard is saying he knows you, the mucking hell he does said Eric show me the mucking bastard muck his mucking walls.
1950		‘Neville Shute’ Town Like Alice 74: Tell the mucking Nip to get those mucking women shifted back so we can get some light.
1957		‘Weldon Hill’ Onionhead (1958) 101: ‘We need a cook [...] Not like ’at muckin’ Red’.
1962		A. La Guma Walk in the Night (1968) 12: You mucking bastard boer with your mucking gun and your mucking bloody red head.
1976		‘In The Melting Pot’ in P. Ashton et al. Our Lives (1982) 175: Hey miss, you can’t hear, turn off that mucking t’ing before me come and mash it ’up.
1985		R.L. Finn Time Remembered 52: ‘Mucking Jew bastard!’ ‘You, mucking bastard!’ said Zaida. ‘I’ll knock your mucking block off.’.