天马行空的英文：
[Literal Meaning]
heavenly horse/cross/sky
A heavenly steed soars across the sky.

[解释]
演讲或诗文气势豪放。

[Explanation]
a powerful and unconstrained style of speaking or writing

[例子]
他在演讲时天马行空，很有气势，但有价值的东西好像不多。

[Example]
His speech was powerful, unconstrained and very forceful, but there seemed nothing valuable.