The uninitiated are always a bit taken aback to see signs advertizing a marché or foire au gras (‘fat market’), but it’s just shorthand for fatted ducks canard gras and geese.
Matières grasses: fats in a food. 0 % de matière grasse means fat free.
In the Catholic church, a jour gras is a day when the faithful can eat meat (as opposed to a jour maigre, a lean no-meat day). Hence Mardi Gras (Fat Tuesday), the last day before Lent and forty jours maigres.
To faire gras is to eat meat, but to faire les choux gras (‘to make the fat cabbages’) means to make headlines in the news.
Image by Joy