It refers to when you look at a person and they look so good it makes you sick, used esp. for females/drag queens/transexual females checking out other females/drag queens/transexual females - but meant completely as an admiration (with a slight connotation of politically acceptable jealousy)
That bitch is SICKENING!!
a person becomes jealous because they perceive someone else as having or being something they don't believe they think they can have or be, so they feel that they are inadequate, that is why they become angry with that individual and try either conspicuously or inconspicuously to sabotage them because they are trying to take something away from them, to feel equal to them.
A lot of unattractive women feel jealousy towards attractive women because they feel that they themselves can never be attractive, so they feel inadequate and consequently, they try to sabotage the attractive girl by excluding her, spreading rumors about her, trying to get her fired, trying to beat her up, etc... but if they got in shape and changed their whole self-image, the attractive chick would no longer be a threat.