A scumbag is a nasty, unpleasant person — someone you think is dirty in character, not in clothes. People spit the word out when they are angry: a cheat, a liar, someone who uses other people. It is very informal and rude. You would not say it to a teacher, in a meeting, or in polite writing. Though actually, it is not the strongest sexual swear word; it is more like calling someone rubbish as a person. Still, it can start a fight. American English uses it a lot; you hear it in films, online, and street talk more than in careful British classroom English. There is no polite cousin with the same bite. If you need a milder word, you want something else entirely, like “a horrible person.”