Nouns ending in -oir are masculine.
The suffix often names a tool, place, or instrument.
Is devoir masculine or feminine?
ledevoir
The rule behind it
100% of -oir nouns are masculine
In context
J'ai la flemme de faire mes devoirs.
I'm too lazy to do my homework.
Je me suis endormi en faisant mes devoirs.
I fell asleep while I was doing my homework.
J'aurai fini mes devoirs à 9 heures.
I will finish my homework by nine.