磨
マ, みが.く, す.る, おさむ
grind, polish, scour, improve, brush (teeth)
JLPT2 G7 S16 F1608
As per 麻# (rub and scrape) +
石
stone → stone used in rubbing or scraping → *rub*; *scrape* → *polish*; *wear down*.
[Full Kanji Etymology List]