Nouns ending in -as are masculine.
Is bras masculine or feminine?
lebras
The rule behind it
100% of -as nouns are masculine
In context
Donnez-lui la main, il vous prendra le bras.
Give him an inch and he'll take a yard.
Montrez-moi votre bras blessé, s'il vous plaît.
Please show me your injured arm.
Je ne peux pas lever mon bras droit.
I can't lift my right arm.