Something (especially a minor thing) which ruins or spoils everything else, or makes it less pleasant; a nuisance or problem; a disagreeable or unpleasant detail.

Etymology

A reference to Ecclesiastes 10:1 in the Bible (King James Version; spelling modernized): “Dead flies cause the ointment of the apothecary to send forth a stinking savour: so doeth a little folly him that is in reputation for wisdom and honour.”

Source

Wiktionary