For 付く(つく):
"This is the intransitive version, though, where something is attached (and you’re not doing the attaching). You can use the く as a hint. You see something attached on the wall. You say “Coo’”
… uh… I do??? 
allllrighty then.
You laugh, but I suppose now you’ll never forget if ku or keru means “to be attached”, huh?
That’s the Koichi dimension. He’s permanently engrained the information into your brain via a low-effort hint. We’re all just pawns on his game of 6-D chess.
Coo’ is a shortening of the word “cool” in some dialects.
Even with that, it’s probably an unusual remark to make when seeing something attached to a wall.
Its definitely 6D chess. By making me post about it here it’s just been doubly ingrained 
Maybe OP says “coo” because they’re a pigeon disguised as a kiwi.
Maybe they can’t hang anything in the Wanikani office because Koichi just stands there cooing at it 
I did figure out it meant “cool”, but only after I noticed the hidden apostrophe.
Why be a wolf in sheeps clothing when you can be a pigeon in kiwi clothing? 