like a heavenly steed, soaring across the skies (idiom)
(of writing, calligraphy etc) bold and imaginative
unconstrained in style
Definitions (粵典–words.hk)
形容詞
創意無限，想像自由奔放 free in thought and behaviour; imaginative; unconstrained; literally: flying horse in the sky
我知呢個計劃係好天馬行空，但我信有一日會成功架。
我知呢个计划系好天马行空，但我信有一日会成功架。
I know this plan seems very far fetched, but I believe it will succeed one day.
創作真係可以天馬行空，冇限制。
创作真系可以天马行空，冇限制。
In fact, artistic creations can express ideas freely without constraint.
Definitions (Wiktionary)
phrase, derogatory, figuratively, idiomatic
speaking in a roundabout, irrelevant or exaggerated way; empty and impracticable talk
phrase, figuratively, idiomatic, of literary composition, usually
powerful and unrestrained in style; bold and imaginative; abounding with novel ideas
沒有什麼能夠阻擋 你對自由的嚮往 天馬行空的生涯 你的心了無牽掛
没有什么能够阻挡 你对自由的向往 天马行空的生涯 你的心了无牵挂
méi yǒu shén me néng gòu zǔ dǎng ， nǐ duì zì yóu de xiàng wǎng ， tiān mǎ xíng kōng de shēng yá ， nǐ de xīn liǎo wú qiān guà Nothing could ever stand, Between you and the free land; In a life of laissez-aller, All your worries were blown away.
phrase, figuratively, idiomatic
progressing swiftly; unrestricted and developing rapidly
Definitions (Kaifangcidian)
（沒有對應漢語詞彙）
Examples (粵典–words.hk)
Cantonese
我知呢个计划系好天马行空，但我信有一日会成功架。
我知呢個計劃係好天馬行空，但我信有一日會成功架。
I know this plan seems very far fetched, but I believe it will succeed one day.
Cantonese
创作真系可以天马行空，冇限制。
創作真係可以天馬行空，冇限制。
In fact, artistic creations can express ideas freely without constraint.
Examples (Wiktionary)
Mandarin
没有什么能够阻挡 你对自由的向往 天马行空的生涯 你的心了无牵挂
沒有什麼能夠阻擋 你對自由的嚮往 天馬行空的生涯 你的心了無牽掛
méi yǒu shén me néng gòu zǔ dǎng ， nǐ duì zì yóu de xiàng wǎng ， tiān mǎ xíng kōng de shēng yá ， nǐ de xīn liǎo wú qiān guà
Nothing could ever stand, Between you and the free land; In a life of laissez-aller, All your worries were blown away.