Definition of "niggling"

My dear Hooker, I am getting very much amused by my tendrils, it is just the sort of niggling work which suits me, and takes up no time and rather rests me whilst writing.

1863, Charles Darwin's letter to J. D. Hooker, 14th of July 1863

Sellafield hasn’t suffered an accident of equivalent scale since the 1957 fire, but the niggling fear that some radioactivity is leaking out of the facility in some fashion has never entirely vanished

December 15 2022, Samanth Subramanian, “Dismantling Sellafield: the epic task of shutting down a nuclear site”, in The Guardian‎

Occasionally a paranoia or slight insecurity that is not based on reality will bring nigglings of that bad time.

2009, Jenny Parratt, Feelings of Change: Stories of Having a Baby, page 10