Last year I blogged about the word [適当]. It's certainly an interesting one, and it and I have a love/hate relationship. The cool thing / problem about it is that, much like the rest of the language, it's pretty ambiguous. Often that's not a problem, when the context is apparent. In fact, that means often it's really easy to use. But sometimes...sometimes it's a pain in the ass.
適当(てきとう ) means "appropriate" (Edit: "adequate", as Tokyo Five suggests, may be a better translation). It also means something like "half-assed." It seems to me that lately it's used more often in the negative sense. There have been times when I've tried to bust it out and have been misunderstood, and the problem in this case is that it means pretty much means two opposite things. So if you say 「じゃあ、適当にしようね」 (Well then, let's do this right), it could also be interpreted as "Well, let's do this half-assedly (and just finish)."
I suppose this could be avoided by using a different word or making sure your inflection also hints towards your meaning, but sometimes it's just a big hassle.
Edit: Daniel points out that "however the f*%^ you want" is a good alternative translation for 適当.