The word dontstirmumpthing itself is a dontstirmumpthing.
Originally, it (likely) meant something far different. However, the experience of the word was undergone during a psychedelic state, and any meaning the word had was lost upon the return to sobriety.
However, the word was so interesting, unique, and generally excellent, that it was decided that the word would be applied to the very nature of the experience that spawned it.
The closest one can possibly come to describing it via the English language is this: The proper "spelling" of the word involves 3d letters, an increase in pitch and a change in the character of the sound on the "mump" syllable (which is not reproducible with a human mouth), and a series of cartoon thumbs (with green nails) animatedly pressing down on a surface on the letters m, u, m, and p.
Bob (not shrooming): What was that, Bob?
Steve (shrooming(on six grams of cubes)): Oh... Uhh... Nevermind. Dontstirmumpthing. So intense. Doesn't matter. Ignore it.