When (one) is least expecting it informal clause
The phrase is sometimes shortened to "When least expected".
Something happens when you are least unexpected or unprepared.
We will take advantage of the enemy's openings to attack when least expected.
A serious problem happened to our project, when we were least expecting it.
If a story, event, or announcement has a sting in its/the tail, it seems pleasant at first but contains a surprising and unpleasant part at the end.
To do something when someone is unprepared
The origin of this expression is not clear.