The adjective emaciated was adopted in the 1660s, and comes from the verb emaciate, which has been around since the 1620s. Emaciated, however, is much more popular with the d than without, as it shows up more than a hundredfold as often as emaciate, and Google autocorrects you if you try to type emaciate. A pity. Anyway, this comes from Latin emaciatus, a past participle of emaciare, a verb meaning "to cause to waste away" much like today. Here, we can eliminate the prefix ex-, meaning "out", leaving us with the root macies, meaning "lean" (as in emaciation brings the leanness out). Macies derives from the verb macer, "to thin", which is from the Proto-Indo-European reconstruction mak, which meant "long" (as in thin things are often long too). Considering that ex- comes from PIE eghs, also "out", if you go as far back as you can, emaciate means "long out"
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