that goes without saying; water is wet
A humorous, idiomatic way to state the obvious. Literally 'when a dog faces west, its tail points east'.
You use 'inu ga nishi mukya o wa higashi' when stating the obvious.
That's like saying water is wet. You don't need to point it out.
A common adjective meaning 'obvious' or 'natural', while 犬が西向きゃ尾は東 is a playful, idiomatic phrase used to humorously call something obvious.
A set phrase based on the obvious fact that a dog's tail is opposite its head; the exact origin is uncertain, but it is used as a humorous idiom for stating the obvious.