A Tanner = Sixpence (Old money- Half a Shilling)
A Rupee was about equal to a shilling, and was divided into sixteen annas. so half a rupee was "eight anna".
I read somewhere that the origin of tanna might actually come from the Gypsy word 'tawno' (although I suspect if that was correct you'd have been aware).
I like the 'eight anna' explanation better though

The Romany word for sixpence is "Shecora" (SP) -I've never written it before.
Mind you, The Roma did come from India originally. Spooky eh?