thick-un

plural thick-uns

1. UKhistoricalobsoleteslangA crown coin; its value, five shillings.

   "That's a good yarn. Now you can't fool me. Tom, I'm not one o' those duds that pay you a thick 'un for an hour in the dark."

   1925 July – 1926 May, A[rthur] Conan Doyle, “(please specify the chapter number)”, in The Land of Mist (eBook no. 0601351h.html), Australia: Project Gutenberg Australia, published April 2019

   1 more example

   Half-a-crown is known as an alderman, half a bull, half a tusheroon, and a madza caroon; whilst a crown piece, or five shillings, may be called either a bull, or a caroon, or a cartwheel, or a coachwheel, or a thick-un, or a tusheroon.

   1859, J.C. Hotten, A Dictionary of Modern Slang, Cant, and Vulgar Words

2. UKhistoricalobsoleteslangA sovereign coin.