IPA: /ˈflætli/
KK: /flætli/
In a way that is clear and without any doubt or emotion.
She flatly refused to accept the offer.
Flatly → It is formed from "flat" (from Old English *flaet*, meaning level or smooth) and "-ly" (meaning in a manner of). The word "flatly" means in a manner that is level or smooth, often used to describe a direct or unequivocal manner of speaking.
Think of something being 'level' or 'smooth' ('flat') and how it can be done 'in a manner' ('-ly'). This helps you remember that 'flatly' means to speak in a direct or straightforward way.