aller à la moutarde

to get told off

In Belgian French this means getting a scolding or dressing down. It is easy to confuse it with the similar sounding idiom about mustard rising to your nose, which instead means losing your temper.

Si tu rentres encore en retard, tu vas aller à la moutarde.

If you come home late again, you will get told off.

Comes from a medieval custom where children were sent to fetch freshly ground mustard before meals, singing cheeky or mocking songs along the way. The phrase drifted towards meaning something secret getting out and mocked, and in Belgian French it settled into meaning being told off.