This word has its origin on the 4chan imageboards, more specificially the "/b/ - Random" imageboard there. The moderators on that board sometimes install wordfilters that change certain words or phrases to other words and phrases when a post is made and much hilarity ensues. A recent example for this was when "FTW" (For The Win) was wordfiltered to "SUCKS". Similarily, at some point the word "cock" was wordfiltered to "harbl" because the moderators got tired of the meme "I'll suck your cock" which of course then always read "I'll suck your harbl". Since this was almost 2 years ago many newer users don't know the origin story (so to speak) and just use it as a random (pun intended) nonsense word. Most recently it has become integrated in the "I'm in ur ______ verb+ing my/your ______" meme.