adj. A person or thing that is insane, crazy, etc. Comes from the colloquial usage of bat shit, meaning the same thing. Guano is the Spanish word for bat droppings.
Person A: Holy shit did you see that guy mumbling to himself?
Guy who likes to get drunk and will be unable to walk or speak at the end of the night. Guano is also pretty much the standard beta kid who has a hard time talking to strangers and rather sits at home playing his videogames. Whenever he acts like a man, it's obvious that he googled how to be one, because he'll start talking about cars, weapons, or other "men stuff". Overall guano is pretty fun when he's around other dudes which is rather suspicious.
A guano is pretty much a drunk beta kid tryin to be cool.