Whose mouth - Psalms 10:7. The apostle has not quoted this literally, but has given the sense. David in the psalm is describing his bitter enemies.
Cursing - Reproachful and opprobrious language, such as Shimei used in relation to David; 2 Samuel 16:5, 2 Samuel 16:7-8.
Bitterness - In the psalm, deceits. The word “bitterness” is used to denote severity, harshness, cruelty; reproachful and malicious words.