Niggling means small but persistently troubling. A niggling doubt, a niggling injury, niggling problems.
The smallness is what makes it hard: a niggling doubt is too slight to act on and too persistent to forget. In sport a niggling injury is one that never quite clears up and never quite stops you playing, which is worse than a clean break in some ways.
1adjectivenigglingSmall but persistently troubling.
“All through the film, I had a niggling feeling that I'd left the oven on.”
niggling is a quality.
Old Norse
hnøggr — stingy; miserly
Norwegian
nigla — to be stingy, to busy oneself with trifles
English
niggle
English, today
niggling
niggle First attested in 1599. Origin unknown, but likely borrowed from dialectal Norwegian nigla (“to be stingy, to busy oneself with trifles”), ultimately from Old Norse hnøggr (“stingy; miserly”), related to Old English hnēaw (“stingy; niggardly”). More at niggard.
Etymology from Wiktionary, CC BY-SA