milligram, (kokuji)
瓱 is a Japanese kokuji that represents the unit milligram. Its meaning is specific to measurement and does not extend beyond this technical sense.
瓱 is a modern Japanese kokuji created by combining 瓦 (gram) and 毛 (one-thousandth), following the pattern of other metric unit kanji. It was coined to represent milligram.
The outer 瓦 stands for gram, and the inner 毛 suggests something tiny (like a hair). Together they form milligram: a thousandth of a gram.
For ミリグラム, the reading is the direct loanword milligram. Picture a tiny milligram weight made of瓦 with a毛 hair on top: milli-gram -> ミリグラム.
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