Not good is it to harp on the frayed string

Don’t keep on talking about the same thing or bringing up the same subject. Literally ‘to harp’ is to play the harp; figuratively ‘to harp on’ is to dwell tediously on a matter. Here ‘frayed ‘means that the harp-string (and the subject) is becoming worn out through too much use.

See also:

• Don't take your harp to the party
• Harp not for ever on the same string
• It is useless to flog a dead horse