Help me understand why 我艹你 means i f*** you? This might seem as an unserious question but i was really stunned the first time I had an argument with a Chinese guy in League of Legends and he said: "我艹你". 

my first thought ofcourse was WTF!?! I grass you? what is he trying to tell me?

so my question is how does i grass you relate to i f*** you? :D
艹 -> 草 -> 操 -> 肏 all with pinyin "cao"

They're all Internet variants on the same word that proliferated through use of computer IMEs. 肏 (the proper character) didn't show up on some earlier IMEs, so 操 was chosen as an alternative for it. From there, 草 became used because it is often the first option that shows up on an IME when people are typing quickly (as is often the case especially in online gaming). From there, 艹 was chosen as a form of self-censorship that is somewhat common on Chinese Internet media.

There are other variants such as 卧槽.