IPA: /ʌnˈflæɡɪŋli/
KK: /ʌnˈflæɡɪŋli/
In a way that shows no signs of weakening or becoming tired; persistently and tirelessly.
She worked unflaggingly to complete the project on time.
Unflaggingly → It is formed from "un-" (meaning not) and "flagging" (from the verb "flag" meaning to tire or weaken, which comes from Old Norse "flaka" meaning to droop or to become weak) and the suffix "-ly" (meaning in a manner of). The word "unflaggingly" means in a manner that does not tire or weaken, showing persistent effort.
Think of 'not' ('un-') being combined with 'tiring' ('flagging') to remember that 'unflaggingly' means to do something without getting tired.