Can be dropped at any time to mean anything you want it to mean. Commonly used by high-level pokemon players and people with a high iq. Used similar to , . or even lol.
1) When one gets high, you tend to go on an adventure (or mission) to do go somewhere, usually in search of munchies. Mish is a short form of mission, and usually refers to this type of situation.
2) Another use of the word mish is to describe something that is far away.