Nouns ending in -is are usually masculine.
Is mauvais masculine or feminine?
lemauvais
The rule behind it
94.1% of -is nouns are masculine
In context
Ken a pris le mauvais bus par erreur.
Ken took the wrong bus by mistake.
Cette voiture est incontestablement en très mauvais état.
That car is no doubt in an awful condition.
Il s'est remis de son mauvais rhume.
He has recovered from his bad cold.