The abbreviation of the phrase Mental Shift.

An M-Shift, or mental shift, is a therianthrope-related occurence wherein a therian feels their mindset, or current mentality, becomes that of their phenotype, or animal.

An M-Shift may be brought upon by many environmental and/or sensory factors, such as weather, certain people, smell, sound, taste, etc.

How intensely an m-shift is felt, and how long they last, vary from therian to therian.

See also: P-Shift, Astral Shift, Suntherian Shift, and Phantom Limbs.
I had an M-Shift about a week back; my first in quite some time.
by Winter Paws January 17, 2007
Short for "mental shift." This is a term therianthropes and otherkin who wish to make excuses for their sometimes bizarre and animalistic behavior use to avoid taking responsibility for their actions; they believe that by claiming that they've "mentally shifted" into their spirit's supposed true form that they should receive pity no matter what they do.
I didn't mean to go on a murderous rampage on the full moon. Please excuse me: I m-shifted into a werewolf mentality and I couldn't help myself!
by Puffing Tom July 14, 2010