'weight-gain' originates from the combination of 'weight' and 'gain', where 'weight' refers to the heaviness of a person or thing, and 'gain' means to increase.

'weight-gain' evolved from the Old English word 'gewæht' for 'weight' and the Old French 'gaigner' for 'gain', eventually forming the modern English term 'weight-gain'.

Initially, it meant 'to increase in heaviness', and this meaning has largely remained the same in modern usage.