avaler des couleuvres

To put up with insults without complaining

Said of someone swallowing their pride and taking insult after insult, the snakes standing for the sly, hard-to-swallow remarks aimed at them.

Au travail, il avale des couleuvres sans jamais répondre à son patron.

At work, he swallows one insult after another without ever answering back to his boss.

One theory ties it to hosts once sneaking snakes into a dish of eels, similar enough in shape that guests swallowed them without noticing. More likely, it plays on an old sense of 'couleuvre' as a sly, underhand remark you have to take without answering back.