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"
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.