What does Theseus name mean?

There is no clear definition of Theseus, but it comes from the same root Word as θεσμός (thesmos) which means "institution".

Theseus was the "founder-king" of Athens (which is odd because he wasn't the first king). Traditionally, he is credited with establishing Athens as a world power at the time. Only Pericles figured more prominently, so "institution" makes sense.