Green’s Dictionary of Slang

tosh n.5

also tush
[? Scot. tosh, smart, neat or dial. toshy, of masculine appearance, hairy-faced]

a form of familiar address to someone whose name one does not know.

[UK]J. Maclaren-Ross ‘The Dark Diceman’ in Bitten by the Tarantula (2005) 205: The spivs [...] hailed him How you doing Tush.
[Ire](con. 1940s) B. Behan Borstal Boy 191: What do you think of it, Tosh?