密封
mì fēng · HSK 7
to seal up
✉️🔒 密封 = 密 tight/airtight + 封 seal = seal up so nothing leaks
密 carries the sense of dense or airtight, and 封 means to seal or close an envelope. Put together, 密封 is sealing something so tightly that no air or content escapes.
密 — 宀 roof + 必 (phonetic) + 山 mountain
Originally referring to a hidden, sheltered place in the mountains, 密 combines 宀 (roof/cover) over 必 (phonetic) with 山 (mountain) at the base. The meaning extended from 'secluded mountain hideaway' to 'dense, close, secret'. Today it commonly means dense, intimate, or secret.
封 — earth 土 doubled (圭) + inch 寸 (hand)
Originally a pictograph of a hand planting a sapling on a mound of earth, marking a boundary. From this came the meaning of granting fiefs to nobles, then sealing or closing off territory. The modern senses 'to seal' and 'envelope' both stem from this idea of enclosing.