omega is the last letter of the greek alphabet. alpha is the first letter. an omega male then is the "last" man. in essence, if you were to rank all the men in the world by their virtues (bravery, intelligence, leadership, generosity, etc.), an omega male altogether would be the worst, hence "last," of all men. none of these other definitions listed on here have much relation to the actual meaning of "omega male" - both as it's been used historically and as it's used currently, particularly the ones that list it as a positive quality and who don't seem to understand the significance of "omega." additionally, the word "omega male" usually refers to a category, rather than a single individual, of "last" men.
if someone says you're an omega male, they're literally saying that not just that every man on earth, save other similarly repugnant omega males, is better (i.e., more generous, braver, etc.) than you, but also, given being "last," that you don't really have these basic virtues at all. it's an insult, not a compliment.
contrast with: alpha male: (the first letter of the greek alphabet, hence "first" man.) effectively, an alpha male is first among men, the best of all men.
the alpha/omega rankings can be used with women as well and are, albeit less frequently.
I can't think of a single positive quality about X - he's a coward, a moron, a sloth, a miser, etc. - he's an omega male.