くそ (kuso, also read as 糞 or クソ) means 'crap'. You can also say the slightly stronger 's-word' version, but as a slang word 'crap' is probably the better translation.
I've long been told that it's unusually difficult to swear in Japanese. You can do it, and there are plenty of derogatory names and sayings to choose from. But the relatively formal nature of the rest of the language makes it highly unusual. Calling someone names is a last resort, for if they've really upset you or disrespected your grandmother.
Calling someone 糞ボケ (kusoboke), crap for brains, is a rare occurrence. The other saying the Japanese have is 糞も味噌も一緒 (kuso mo miso mo issho), which implies that someone can't tell the difference between crap and miso paste. Not exactly light-hearted japery, this.
On top of that, not all insults translate into English well. Yeah, you keep walking, 'woman of dubious morals'! And some of them are just a less nice way of saying 'you'. The great thing about 'kuso' (if you can call it a great thing) is that it's a bit more versatile as a word.
For example, 下手糞 (hetakuso) means 'extreme clumsiness'. People will yell this at you if you're really bad at something. So it's just like 'being crappy' at it.
If the weather's naff, you can say that it's クソ暑い (kusoatsui) - crapping hot. Crap, it's hot. It's hot as crap, etc. 'Kuso' can be tacked onto the start of descriptive words like this as a slang-y substitute for 'very'. 'totally' or 'completely'. It's a way to say you're very engrossed in studying, or that something isn't interesting or funny at all.
I've tried using this myself to describe things that really suck, but it doesn't feel quite the same. I guess I'm just not good at swearing in Japanese. 糞.