tube, n.² — Green’s Dictionary of Slang

tube n.²

[a tube is hollow inside, like the person’s brain; or use of tube n.¹ (1) as generic insult]

(Scot.) a person, usu. with a suggestion of stupidity.

1997		Share Slanguage.
1998		I. Welsh Filth 32: You think [...] any ay the cunts on the promotion board’ll listen to that tube.
2006		I. Rankin Naming of the Dead 2007 374: Fuck off. Six in the top corner, watch this, ya choob.
2012		(con. 1980s) I. Welsh Skagboys 85: Steady, ya fuckin choob.