Zelo was rejected multiple times from companies because of his young age.

But he kept on trying and trying to be his best so that he would get accepted despite his age and he was finally accepted by TS.

That's how he got his nickname 'Zelo'.

'Zelo' comes from the Greek God 'Zelus'[I think thats how you spell it. xD], and Zelus is the greek god of jealousy, rivalry etc. meaning that Zelo strives to be the best.