*Note that “Faire la grasse matinée” is often shortened to “faire la grasse mat”.
Literally: to have a fat morning
Meaning: to sleep in
Origins: This expression is an old one, going back to at least the 16th century, when dormir la grasse matinée was used. Despite a popular misconception about the physical effects of staying in bed all morning, linguists believe that the grasse has more to do with the ‘thickness' or ‘softness' of deep sleep, according to Le Figaro.