Afire means burning with real flames, or glowing as though flames were inside. The barn was afire, eyes afire, set afire.
It is predicative only — things are afire and there are no afire things — and it is literary where on fire is ordinary. The figurative sense is the commoner one now: afire with ambition, afire with anger. Note aflame and ablaze, which are built the same way from the same old prefix meaning on.
1adjectiveafireBurning, or glowing as though burning.
“The sunset set the whole sky afire with red and gold.”
synonyms: ablaze, aflame
afire is a state of burning.
Middle English afire is a-, an old form of on, plus fire. Aflame, ablaze and asleep are formed the same way.
Middle English
afire
English, today
afire
13thc., from Middle English afire, equivalent to a + fire. Figurative usage from late 14thc.
Etymology from Wiktionary, CC BY-SA