coup de main - noun
kü-də-ˈmaⁿ
Definition of coup de main : an attack that achieves complete surprise.

Halsey is teasing something lucky with a mysterious website...

Halsey is possibly teasing something over on her website 'formylasttrick'... there are now new tabs to press on including 'drafts', 'letters', and 'lucky'.

Eagled-eyed fans are predicting that the word 'Lucky' might have something to do with new music...