Superlative form of shaggy, meaning having the longest, roughest, and untidiest hair or fur.
The word shaggiest makes people think of unkempt, messy, or wild things.
Examples:
The shaggiest dog I've ever seen had hair all the way to the ground.
Her shaggiest coat was perfect for the cold winter.
The shaggiest rug in the house was also the softest.
That is the shaggiest sheep I've ever seen!
The shaggiest tree in the forest looked like a big green monster.
The shaggiest dog in the world won a prize for being so shaggy.