Être à l’ouest, literally “to be in the west”, is very useful if you are feeling tired and a bit off. The expression means to be spaced out, to be unable to pay attention, to not be with.
The expression dates back to the 20th century. Its origin is not very clear. However, the most plausible explanation is that it comes from the English expression “to go west” which during WWI meant “to die” or “be killed”.