Лох
Лох (n., m.) - coarse and slangy, contemptuous - ignoramus, a person who knows and understands nothing in particular subject or in general. Foolish or stupid, amateurish person. Fool, idiot, common person.
Лохануться (v., perf.asp.) - coarse and slangy, contemptuous - to do a foolish or stupid act.
Лоховство (n., n.) - coarse and slangy, contemptuous - description of something of bad taste, demonstrating the lack of education, foolish, amateurish.
Examples:
1). Yesterday on MTV (talk-show):
The guest says: Я полный лох в автомобилях. (I am a complete ignoramus where cars are concerned.)
The host replies: Лучше быть лохом в автомобилях, чем лохом в автобусе! (Better to be an ignoramus in cars then a fool in a bus!)
The joke is based upon the difference between two nuances in the meaning of the word "лох": the guest means he doesn't understand much in cars, and the host means that it's better to know nothing in cars but own and drive them, then to be a common fool who uses public transportation)

2). На дороге: (At the road:)
- По той или этой улице поедем? (Which way to choose, this or that?)
- Давай по этой. (Let's go this way.)
Через 5 минут упираются в огромную пробку. (In 5 min, they get stuck in a huge traffic jam.)
- Что-то мы с тобой лоханулись, надо было ехать в другую сторону. (Seems like it was a foolish decision, should have taken another way.)