Final answer:
MSM stands for Mirrored Self Misidentification, a psychological disorder where a person misidentifies their reflection in the mirror as someone else, often associated with dementia or brain injuries.
Step-by-step explanation:
The acronym MSM typically stands for Mirrored Self Misidentification, which is a rare delusional misidentification syndrome. In this condition, a person believes that the reflection in the mirror is another person, often a stranger or someone they used to be, instead of their own reflection. This psychological condition is connected with various forms of dementia, such as Alzheimer's disease, as well as certain types of brain injuries.