KanjiJLPT N1
Tail

tail, end, counter for fish, lower slope of mountain

Mnemonic

A flag atop the castle wall ripples in the wind, but as you climb closer you see it's covered in thick fur. It's no flag at all—it's the enormous tail of a sleeping dragon draped over the battlements, lazily swishing back and forth in the breeze.
Additional thoughts
Picture the flag shape (尸) as the silhouette, then the fur (毛) texture reveals the truth: it's a giant tail. The flag's flowing motion mirrors how a tail sways.
Quick recall
A flag made of fur is really a dragon's tail draped over the wall.

Details

The keyword for 尾 is tail. This kanji refers to the tail of an animal, the rear appendage that extends from the body. By extension, it can also mean the end, trailing part, or bottom of something, much like how a tail is found at the rear. It carries the sense of something that follows behind or comes last in a sequence.
On'yomi
Kun'yomi