Getae and Dacians? Are they the same? Or is this unknowable?

[Justin of the New Yorkii]

Its not sure from where this name show up, but for sure Getae and Dacians was the same peoples, even if some said that regional groups of one and the same peoples.

And that is precisely what bothers me. We have an idea where the term 'Daci' came from, but not Getae, and there also happens to be two other ancient peoples with similar ethnonyms, the Thyssagetae and Massagetae.
But it appears that no historians, modern or ancient, know where the term 'Getae' originated, as far as I know.