Green’s Dictionary of Slang

rosie loader n.

also rosie loder, rosy loader, rosy loder
[rhy. sl.]

soda water.

[Scot]Eve. Teleg. (Dundee) 3 Aug. 4/1: He was having a 'Jack and Rosie' [...] 'Jack-the-Dandy' (brandy), 'Rosie Loader' (soda).
[UK]J. Franklyn Dict. of Rhy. Sl.
[UK]Partridge DSUE (8th edn) 989/1: C.20.