late 13c., "flaming, full of fire," from Middle English fier "fire" (see fire, n.) + -y(2). The spelling is a relic of one of the attempts to render Old English "y" in fyr in a changing system of vowel sounds. Other Middle English spellings include firi, furi, fuiri, vuiri, feri. From c.1400 as "blazing red." Of persons, from late 14c. Related: Fieriness. As adjectives Old English had fyrbære "fiery, fire-bearing;" fyren "of fire, fiery, on fire;" fyrenful; fyrhat "hot as fire."