Often used to stress the frequency of an action, usually out of frustration or exasperation. You've come in late for the thousandth time!
Pro Tip 2/3
Not Literal
When 'thousandth' is used figuratively, it does not denote the exact number but implies a lot. This feels like the thousandth time you've changed your mind
Pro Tip 3/3
Emphasize Repetition
Can underline that something has happened too many times, often more than what's reasonable. He's apologized for the thousandth time!