IPA: //ʌnˈfruːt.fəl.i//
KK: /ʌnˈfrutəfli/
In a way that does not produce good results or benefits.
They worked unfruitfully for hours without making any progress.
Unfruitfully → It is formed from "un-" (meaning not) and "fruitful" (from Old English "fruhtfull", meaning producing fruit or beneficial). The word "unfruitfully" means not producing fruit or not yielding beneficial results.
Think of 'not' ('un-') producing anything beneficial ('fruitful') — that's why unfruitfully means not yielding good results.