It's not a real word. It's an exaggeration of the word "ridiculous". "Ridiculous" means "absurd" or "crazy". "Redonkulous" means MUCH MUCH MORE ridiculous. The size of that (big) fish is ridiculous. But the size of that (huge) fish is redonkulous!

Слово, по-видимому, «ridiculous», что значит «смешной» в том смысле, что это глупо или непонятно, а не весело.

Lol. It isn't a word, but it's meant to sound like a ridiculous way of saying rediculous, just to emphasise how ridiculous something is.