Actually it comes out of 2 words: "Εν+τάξη" which means "in order"...Ola Kala : Όλα Καλά.
In modern Greek it evolved to "εντάξει". "ει" is used for adverbs, even though "εντάξει" is not one.
Here is an example from the US-English dialect: "want to" becomes "wanna". "Εντάξει" is also a mistake that we got used to say and write. No one wondered why it happens, as it happens with half of the modern Greek words.