He is good at flattery.
That sounds like flattery.
He's good at flattering his boss.
That shop clerk is so good with flattery that I end up buying something.
She showered the client's president with flattering remarks.
世辞 (お世辞) is insincere praise meant to please, while 悪口 is speech that criticizes or badmouths someone.
お世辞 is the polite, everyday form almost always used in speech, while 世辞 alone sounds abrupt or literary and is rarely used by itself.
世辞 (flattery) and 世事 (worldly affairs, everyday matters) share the identical reading せじ but have completely unrelated meanings.
Standard kanji spelling. Usually appears with the honorific prefix お as お世辞.
Kana spelling is common in everyday writing, especially when the word is used without the prefix お.
The exact derivation is uncertain. The kanji 世 (world, society) and 辞 (word, speech) suggest 'social words,' fitting the meaning of flattery or polite talk.