expression
this is my chance; what an opportunity
An exclamatory phrase used when a favorable opportunity arises, often followed by と or とばかりに to indicate seizing the moment.
これ幸いとばかりに、彼はその場を離れた。
Seizing the opportunity, he left the scene.
相手のミスを見て、これ幸いと攻め込んだ。
Seeing the opponent's mistake, they took the chance and attacked.
Compound of これ (this) and 幸い (good fortune, opportunity). The exact historical derivation is uncertain, but the phrase is conventionally used to express seizing a chance.