Here's why:
* Two words combined: It's formed by combining two separate words, "shag" and "rug," to create a new, singular meaning.
* Describes a single thing: It refers to a specific type of rug with a long, fluffy pile, not two separate things.
Compound nouns can be written as one word (e.g., "toothbrush"), two words (e.g., "shag rug"), or hyphenated (e.g., "high-school").