Don't worry. It doesn't mean I'm about to come up with some famous last words.
That is the good news.
Basically, it's a comeback for when someone says something that might well tempt (or bait) fate!
Well, that's it's idomatic meaning.
Use in Normal SpeechEdit
When warning of possible trouble.
"I know that's what they want to happen, but they should be realistic...what they're saying could well be famous last words!"