Nouns ending in -eau are usually masculine.
Is manteau masculine or feminine?
lemanteau
The rule behind it
93.1% of -eau nouns are masculine
In context
J'ai acheté ce manteau à bas prix.
I bought this coat at a low price.
Il y a des boutons sur le manteau.
There are buttons on the coat.
Un des boutons de mon manteau est arraché.
One of the buttons has come off my coat.