1) A loser, often of near biblical proportions; a person classified as such tends to exhibit a plethora of annoying qualities such as a tendency to high-handed moralising, offering up unsolicited non-sequiturs out of the blue and issuing commandments on how to do things, but without having been given the mandate to do so by neither authority on high nor by election of his peers.
2) Sometimes slight homosexual connotations are attached to the meaning, given its etymological origins: homo was contracted to Mo, then extended to Moses to render a more euphonious word and offer some degree of stealth due to possible censorship and slander concerns.
"Hey, guys, you really shouldn't be eating all that fast food. It could kill you."
"Shut up, don't be such a moses"
or, singing along to a Nelly song:
"Why do I live this way? Hey, must be a moses"