Idiomatic expression meaning a final, decisive resource kept in reserve. A non-standard variant of 伝家の宝刀, often used humorously or emphatically.
See also: 伝家の宝刀
He drew his trump card as a last resort.
交渉が難航したとき、社長は天下の宝刀を使った。
When negotiations stalled, the president used his ultimate trump card.
The standard form of the idiom, literally 'family heirloom sword'. 天下の宝刀 is a non-standard variant that replaces 'family heirloom' with 'world/realm', often used for humorous or exaggerated effect.
A non-standard variant of the idiom 伝家の宝刀 (one's family heirloom sword, i.e., a last resort). The substitution of 天下 (the world, realm) for 伝家 (family heirloom) is likely a playful or emphatic alteration, not a historically established form.