Etymology
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magma (n.)

mid-15c., "dregs, any crude mixture of organic matter," from Latin magma "dregs of an ointment," from Greek magma "thick unguent, ointment," from root of massein "to knead, mold," from PIE root *mag- "to knead, fashion, fit." Geological meaning "molten or semi-molten rock" is by 1859. Related: Magmatic.

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*mag- 
also *mak-, Proto-Indo-European root meaning "to knead, fashion, fit." It forms all or part of: amass; among; macerate; magma; make; mason; mass (n.1) "lump, quantity, size;" match (n.2) "one of a pair, an equal;" mingle; mongrel.

It is the hypothetical source of/evidence for its existence is provided by: Greek magis "kneaded mass, cake," mageus "one who kneads, baker;" Latin macerare "soften, make soft, soak, steep;" Lithuanian minkyti "to knead;" Old Church Slavonic mazo "to anoint, smear;" Breton meza "to knead;" Old English macian "to make, form, construct, do," German machen "to make;" Middle Irish maistir "to churn."
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