Green’s Dictionary of Slang

tonk adj.

(UK teen) very large.

[US]BlazinParadise ‘Blazing Squad Language’ 🌐 Tonk – Massive [...] Tonk arse – Big bum.
[UK]G. Malkani Londonstani (2007) 63: Guys who’re fitter an tonker an better dressed than me.
[UK]A. Wheatle Dirty South 59: A tonk looking brother was approaching us [...] This guy had He-Man muscles on his neck and probably owned an eight-pack.
[UK]Guardian 27 June 🌐 [T]oday’s uber-tonk males wear their six-packs like beautiful, pointless feathers: this is a cosmetic muscularity, rather than a functional one.

In compounds

tonked up (adj.)

muscled.

[UK]G. Malkani Londonstani (2007) 190: Being tonked up just made guys carry on actin like foolish cavemen.