They became brothers-in-arms during their startup journey.

brothers 是 brother 的复数形式，意思是"兄弟"。 in-arms 由介词 in（在...中）和名词 arms（武器）组成，字面意思是"在武器中"。 brothers-in-arms 是一个固定短语，指"战友"或"并肩作战的伙伴"，比喻共同经历艰难挑战的亲密伙伴关系。