the whole country; the world; the public; the ruling power; having one's own way
Classical compound literally meaning 'under heaven'. Used in historical or literary contexts to refer to the realm, the nation, or the world under a ruler. The sense 'having one's own way' is rare and appears in set expressions like 天の下知らぬ顔.
天の下、王の命に従わぬ者はいなかった。
Under heaven, there was no one who disobeyed the king's command.
He acted as if he owned the world.
Compound of 天 (あめ, 'heaven') + の (possessive particle) + 下 (した, 'under'). The exact historical derivation is uncertain, but it follows a common pattern for classical Japanese expressions.