noyer le poisson

to deliberately make an issue unclear

It means obscuring a discussion with irrelevant details so the real issue disappears.

Pendant l'entretien, il a noyé le poisson au lieu de répondre clairement.

During the interview, he muddied the issue instead of answering clearly.

In the 19th century, it was fishing jargon meaning to tire out a fish caught on the hook so you can pull it out more easily. By extension, it became figurative: to “muddy things” and create confusion to dodge a question or avoid addressing the point