manger sur le pouce

To eat something quickly, often standing up

The thumb once served as a makeshift chopping board, propping up bread or sausage while you ate standing up in a rush.

Je n'ai pas eu le temps de déjeuner, alors j'ai mangé sur le pouce entre deux réunions.

I didn't have time for lunch, so I grabbed something quick between two meetings.

The thumb used to double as a support for cutting a slice of bread or sausage while eating standing up, without sitting down at a table, which is where the image comes from.