Etymology
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mordant (adj.)

late 15c., "caustic, biting, severe" (of words, speech), from Old French mordant, literally "biting," present participle of mordre "to bite," from Latin mordēre "to bite, bite into; nip, sting;" figuratively "to pain, cause hurt," which is perhaps from an extended form of PIE root *mer- "to rub away, harm." Related: Mordantly.

The noun is first attested in a now-obsolete or archaic sense of "ornamented hooked clasp of a belt or girdle" (mid-14c.), from Old French mordant in this sense. In dyeing, "substance used in fixing colors," it is attested by 1791; as an adjective in dyeing, "having the property of fixing colors," by 1902. Related: Mordancy; mordantly.

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smirch (v.)

late 15c., "to discolor, to make dirty," of uncertain origin, perhaps from Old French esmorcher "to torture," perhaps also "befoul, stain," from es- "out" (see ex-) + morcher "to bite," from Latin morsus, past participle of mordēre "to bite" (see mordant). Sense perhaps influenced by smear. Sense of "dishonor, disgrace, discredit" first attested 1820.

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breakfast (n.)

"first meal of the day," mid-15c., from the verbal phrase; see break (v.) + fast (n.). For vowel shift, see met (v.). An Old English word for it was undernmete (see undern), also morgenmete "morning meal."

Spanish almuerzo "lunch," but formerly and still locally "breakfast," is from Latin admorsus, past participle of admordere "to bite into," from ad "to" + mordēre "to bite" (see mordant). German Frühstück is from Middle High German vruostücke, literally "early bit."

In common with almuerzo, words for "breakfast" tend over time to shift in meaning toward "lunch;" compare French déjeuner "breakfast," later "lunch" (cognate of Spanish desayuno "breakfast"), from Vulgar Latin *disieiunare "to breakfast," from Latin dis- "apart, in a different direction from" + ieiunare, jejunare "fast" (see jejune; also compare dine). Greek ariston in Homer and Herodotus was a meal at the break of day but in classical times taken in the afternoon.

The long/short vowel contrast in break/breakfast represents a common pattern where words from Old English have a long vowel in their modern form but a short vowel as the first element of a compound: Christ/Christmas, holy/holiday, moon/Monday, sheep/shepherd, wild/wilderness, etc.

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*mer- 

Proto-Indo-European root meaning "to rub away, harm." Possibly identical with the root *mer- that means "to die" and forms words referring to death and to beings subject to death.

It forms all or part of: amaranth; ambrosia; amortize; Amritsar; immortal; manticore; marasmus; mare (n.3) "night-goblin, incubus;" morbid; mordacious; mordant; moribund; morsel; mort (n.2) "note sounded on a horn at the death of the quarry;" mortal; mortality; mortar; mortgage; mortify; mortmain; mortuary; murder; murrain; nightmare; post-mortem; remorse.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mrnati "crushes, bruises," mriyate "to kill," martave "to die," mrta- "died, dead," mrtih "death," martah "mortal man," amrta- "immortal;" Avestan miriia- "to die," miryeite "dies," Old Persian martiya- "man;" Hittite mer- "to disappear, vanish," marnu- "to make disappear;" Armenian meranim "to die;" Greek marainein "to consume, exhaust, put out, quench," marasmus "consumption," emorten "died," brotos "mortal" (hence ambrotos "immortal"); Latin mors (genitive mortis) "death," mori "to die;" Armenian merani- "to die;" Gothic maurþr, Old English morþ "murder;" Old Irish marb, Welsh marw "dead;" Lithuanian mirti "to die," mirtis "death;" Old Church Slavonic mreti "to die," mrutvu "dead;" Russian mertvyj, Serbo-Croatian mrtav "dead."

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