Meaning of “Tsuyotsuyo(つよつよ)”

“Tsuyotsuyo(つよつよ)” means “super strong” or “having an extraordinary ability” kind of…
This is an slang.
In this case “tsuyo(つよ)” is an abstraction for “tsuyoi(強い)” which means strong.

Example Conversation

A: Why can she endure such a terrible environment like that?
B: Yeah, I think her mental is tsuyotsuyo.

A: あの子なんであんな環境で頑張れるんだろ
B: ほんとだよね、メンタルつよつよなんだと思う