Cronssant is a colloquial name for the Necron flyers introduced in Matt Ward's 5th edition Codex for Warhammer 40,000, the Night Scythe and Doom Scythe. They are referred thusly because they look like giant, flying, metal croissants (the delicious French pastry!) and they were overpowered enough to be considered cheese, making the nickname even more fitting.
Yes, we agree it's a stupid name. But then somebody went and made Cronssants out of actual croissants.