IPA: /ˈdɑː.lɪŋ.li/
KK: /ˈdɑr.lɪŋ.li/
In a way that shows affection or endearment.
She spoke darlingly to her pet, making it feel loved.
Darlingly is formed from "darling" (from Old English "deorling", meaning beloved or dear) and the suffix "-ly" (meaning in a manner of). The word "darlingly" means in a manner that is beloved or dear.
Think of someone acting in a way that shows they are 'beloved' ('darling') — the '-ly' indicates the manner of that action.