harp on one string
To dwell on a single subject with disagreeable or wearisome persistence.
VERB + HARP
set, tune
The musician tuned her harp carefully before the concert began.
ADV
always, constantly
My mother constantly harps on about how expensive my phone bill is every month.
still
PREP
about
PHRASES
keep harping on
My brother keeps harping on about how I never help with the housework.