пидар literally means "fag". In russian, calling somebody that does not literally mean "homosexual", but merely means a somewhat harder form of moron. Yes, it is definately obsene, I know when I was younger I called more people that, than I do now, as I realize that the word is rather harsh.
- Petya u menya zhevachku ukral - Nu vot on i pidor !
If you are teaching an ESL class to youngsters, a lot of times the word can be used just because they are pissed at you and want to use a "cool" harsh word with their peers. I know when I was 12 in an ESL class, I would of done that.