"Thank you very much!"
Translation:Tack så mycket!
As a prefix, jätte- means "huge". We use it all of the time for loads of things. So for instance:
- tack = thanks
- tack så mycket = thanks a lot
- tack så jättemycket = thank you so terribly much
Sounds silly in English but it's a normal if occasionally colloquial way of expressing largeness.
It's also a word of its own, in which case it means "giant" - as in the noun, a huge person.
It is reasonably common. At least among adults/elders. :) Means something along the lines of "you should have thanks"/"thanks given to you".
It would be used to specify that you're pleased with that particular person. Rather than just a common appreciative thanks given out of courtesy. So this and "tusen tack" has a bit of the same usage.
Additionally it can be used to rethank someone. So... trying for a scenario here... but cannot think of a good one. But basically if someone says thanks to you, but you want to thank them you could say "tack ska DU ha" with an emphasis on the you.
Hope I didn't make things more confusing!