'comrade-in-arms' originates from the combination of 'comrade' and 'in arms', where 'comrade' meant 'companion' and 'in arms' referred to 'military service'.

'comrade' changed from the French word 'camarade' and eventually became the modern English word 'comrade'.

Initially, it meant 'a companion in the military', but over time it evolved into its current meaning of 'a fellow soldier or someone who has shared difficult experiences'.