천만다행이 · 천만다행을 (often 천만다행이에요, 천만다행으로)
a great stroke of luck, a huge relief; thank goodness
Example
큰 사고였는데 아무도 안 다쳐서 천만다행이에요.
It was a big accident, but thank goodness nobody got hurt.
Word parts
천 thousand+만 ten thousand+다 many+행 luck
千萬 'ten million' + 多幸 'good fortune' — lucky ten million times over.
Good to know
A stronger 다행이다; said after a near miss. 불행 중 다행 means 'a blessing in disguise / it could have been worse'.