To Brush One's Teeth
expression, godan verb
磨く is "to polish" or "to shine." If you're doing that to your teeth you are brushing them. That's why this is to brush one's teeth.
The reading comes from 歯 and 磨く, put together.
「どうして歯を磨かないの？」「だって、食べ物の味が口に残っているのが好きなんだもん。それっていけないことかしら？」
"Why don’t you brush your teeth?" "Because I like how the taste of food stays in my mouth! What’s so wrong with that?!