What does "that ship has sailed" mean?

The opportunity has passed - it's too late to take advantage of it now.

"I thought about applying for that role, but that ship has sailed."
"We could have merged with them two years ago, but that ship has sailed."
"I'm afraid that ship has sailed - the deadline was last week."

A ship that has left the harbour cannot be caught once it's gone. Missing the departure means missing the opportunity entirely.

There was a window of opportunity, and it closed. The speaker acknowledges this with resignation - not necessarily regret, just acceptance that the moment is gone.

Resigned, matter-of-fact. Can be said about your own missed opportunities or about explaining to someone else why something is no longer possible.

• "The moment has passed." - plain
• "It's too late now." - direct
• "The window has closed." - slightly more formal
• "You missed the boat." - similar idiom, same meaning (see: miss the boat)
• "It's water under the bridge." - different - refers to past things that can't be changed, not just opportunities

Casual to neutral. Common in conversation and informal professional communication.