It seems to me that "tja" is more a typo for "ja" than a word by its own.
I have looked in a number of websites, the only satisfying one was Wiktionary but it has nothing about its etymology as a German word.
How did the word "tja" originate?
German Language Stack Exchange is a bilingual question and answer site for speakers of all levels who want to share and increase their knowledge of the German language. It's 100% free, no registration required.Sign up to join this community
"tja" is an interjection (Interjektion). Like "oh", "ah" or "pst". I wouldn't consider it as a word which has developed from something.
You could also say "nun ja", but what does that really mean? Just filling words or a filling sound to bridge the silence till your thoughts caught up.
I doubt it originated from a typo, because it is used mainly in spoken German. It get from there into the literary language. And it sounds better as "uhm" "ähm" or "äh", but fulfills the same purpose.