密 means “secret, dense, close”

みつ · mitsu

Also: secrecy, density (pop), minuteness, carefulness

Kun'yomi (Japanese reading)
ひそか
On'yomi (Chinese reading)
ミツ

JLPT N1 · Taught in grade 6 · 11 strokes

The story

Under a roof (宀), certainly (必) hidden deep in the mountains (山): close-packed and out of sight. 密 is secret, as in 秘密, and dense: 密度 is density, and in 2020 Japan's 三密 warnings said to avoid closed, crowded, close-contact places.

Used in 秘密 (himitsu, secret), 密度 (mitsudo, density), and 密接 (missetsu, close/intimate)

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Readings from KANJIDIC2, © EDRDG, CC BY-SA.