天马行空? [ tiān mǎ xíng kōng ]?
1. The divine horse gallops in the air, mostly describing poetry, calligraphy, words and deeds, etc. as bold and unrestrained.
2. Describes speaking and doing things that are irrelevant.
Source
Yuan Dynasty Liu Tingzhen's "Preface to the Collection of Poems of Sa Tianxi": "The reason why it is deified and transcends all other forms is almost like a horse moving in the sky and making extraordinary steps."