It sounds very different, but if you break it down the building blocks are quite familiar.
The Old Norse "ekki", which you still see in modern Icelandic, was formed from "eitt" (one) and the negative suffix "-ki". Literally meaning "not one", and also used in the sense of "nothing".
In greek there is the word "όχι" pronounced "óhi" with the " h " pronounced much stronger than in english. This word means "no" and it is also very different from all orher languages. X in greek is considered a softer issue of k. For me ikke sounds like the greek óhi with nowergian accent, like a norwegian trying to say the ohi when he listens it for first time. Taking into account that greek language is one of the most ancient in the world and the fact that greeks had made trips in these areas since ancient years, it is possible that the origins of this word may be from greek ohi. Who knows? For me it works to remember that word in this way.