Etymology
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ferrous (adj.)
"pertaining to or containing iron," 1865, from Latin ferreus "made of iron," from ferrum "iron" (see ferro-). In chemistry, "containing iron," especially with a valence of two. Contrasted with ferric.
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ferro- 
before vowels ferr-, word-forming element indicating the presence of or derivation from iron, from Latin ferro-, combining form of ferrum "iron," which is of unknown origin. Possibly of Semitic origin, via Etruscan [Klein]; Watkins suggests "possibly borrowed (via Etruscan) from the same obscure source as OE bræs "brass." Also sometimes especially indicative of the presence of iron in the ferrous state; ferri- indicating iron in the ferric state.
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