Used in messaging or email to communicate surprise or exasperation beyond that which would warrant a simple "OMG". Abbreviation of "Oh My God, Meredith!", an exclamation used (frequently) by contestant Gretchen on Season 7 of The Amazing Race. The "Meredith" named in the phrase is Gretchen's husband and race partner ... Meredith.
OMGM! We don't have any milk, so I had to have water on my corn flakes AGAIN!