It's not said seriously

JEST

Think of a lighthearted remark often made to amuse others, like a playful tease or witty comment. Shakespeare’s fools were known for this.

The clue "It's not said seriously" hints at a lighthearted or playful remark rather than something sincere. "Jest" fits perfectly because it means a joke or a teasing comment—something not meant to be taken literally. The wordplay is straightforward here, relying on the direct definition of "jest" as the opposite of serious speech. It’s a common crossword trick to use simple synonyms for misdirection.