"nettle-rash," medical Latin, from Latin urtica "nettle, stinging nettle" (figuratively "spur, incentive, stimulant), from urere "to burn," from PIE root *eus- "to burn" (see ember) + abstract noun ending -ia. Related: Urticarial.
"small, live coal," Old English æmerge "ember," merged with or influenced by Old Norse eimyrja, both from Proto-Germanic *aim-uzjon- "ashes" (source also of Middle Low German emere, Old High German eimuria, German Ammern); a compound from *aima- "ashes" (from PIE root *ai- (2) "to burn;" see edifice) + *uzjo- "to burn" (from PIE root *heus- "to burn;" source also of Sanskrit osati "to burn, scorch," usna- "hot;" Greek euo "to singe;" Latin urere "to burn, singe;" Old Norse usli, Old English ysle "hot ashes," Old Norse ysja "fire"). The -b- is unetymological.
word-forming element in names of countries, diseases, and flowers, from Latin and Greek -ia, noun ending, in Greek especially used in forming abstract nouns (typically of feminine gender); see -a (1). The classical suffix in its usual evolution (via French -ie) comes to Modern English as -y (as in familia/family, also -logy, -graphy). Compare -cy.
In paraphernalia, Mammalia, regalia, etc. it represents Latin or Greek -a (see -a (2)), plural suffix of nouns in -ium (Latin) or -ion (Greek), with formative or euphonic -i-.
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Definitions of urticaria from WordNet
an itchy skin eruption characterized by weals with pale interiors and well-defined red margins; usually the result of an allergic response to insect bites or food or drugs;