Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are the first word in a sentence.
* Proper nouns refer to specific people, places, or things. They are capitalized.
"Word drawer" is a general term for any drawer that holds words. It doesn't refer to a specific drawer, so it's a common noun.
Let me know if you have other words you want to check!