Nouns ending in -esse are feminine.
The suffix often marks an abstract quality.
Is fesse masculine or feminine?
lafesse
The rule behind it
100% of -esse nouns are feminine
In context
Ne déconne pas ! Il en va de mes fesses.
Don't screw up. My ass is on the line.
Sami a claqué les fesses de Layla.
Sami smacked Layla on the butt.
Bouge tes fesses et fais quelque chose !
Get off your butt and do something!