Turbocharged Supercharged Intercooled is unfortunately not the correct abbreviation (although I sometimes wish it was). Even the Golf GTI is a TSI.
The commonly suggested answer on the Internet is Turbo-Stratified Injection
Perhaps there's also a longer unpronounceable German version as well just as DSG = Direct Shift Gearbox = a type of Doppelkupplungsgetriebe (Dual Clutch Gearbox) :)