'fire-dog' originates from English, specifically a compound of 'fire' and 'dog', where 'fire' meant 'fire' and 'dog' in this sense meant 'a metal peg, clamp or support (a device that holds things in place)'.

'dog' in the sense of a metal device is attested from Middle English usage meaning 'a clamp or peg'; combining it with 'fire' produced 'fire-dog' (also written 'firedog' or 'fire dog') to denote an iron support in a hearth by Early Modern English.

Initially, the element 'dog' referred more generally to a peg, clamp, or holding device; over time the compound came to mean specifically 'an iron support for logs in a fireplace.'