Green’s Dictionary of Slang

shower adj.

(UK black) important, ruthless.

[UK]G. Krauze What They Was 210: Being shower is having mad heart, going to mad lengths, never backing down — it’s having no heart in fact.

In compounds

showerman ()

(UK black) a major figure in the local underworld or creative culture.

Urban Dict. 7 May 🌐 A showerman is someone that is ‘big’ in what he is doing, big on road or in music (as MC for example). Originated from the grime scene and originally came from East London.
[UK]G. Krauze What They Was 183: They’re talking about some next showerman called Magic [...] and I start thinking how these names hold power [ibid.] 210: Man like Bugz Bunny are real showerman though. Licking man down with no regrets.