Definition & Meaning of "the last thing on one's mind"in English

What is the origin of the idiom "the last thing on one's mind" and when to use it?

The exact origin of the idiom "the last thing on one's mind" is uncertain, but it is believed to have originated in the United States in the early 19th century. There are a few different theories about its origins, but the most likely is that it refers to something that is so unlikely or improbable that it is not even worth considering.