Etymology
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warranty (n.)
mid-14c., legal term for various types of clauses in real estate transactions, from Anglo-French and Old North French warantie "protection, defense, safeguard" (Old French garantie), from warant (see warrant (n.)).
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gu- 
because g- followed by some vowels in English usually has a "soft" pronunciation, a silent -u- sometimes was inserted between the g- and the vowel in Middle English to signal hardness, especially in words from French; but this was not done with many Scandinavian words where hard "g" precedes a vowel (gear, get, give, etc.). Germanic -w- generally became -gu- in words borrowed into Romance languages, but Old North French preserved the Frankish -w-, and English sometimes borrowed both forms, hence guarantee/warranty, guard/ward, etc.
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*wer- (4)

Proto-Indo-European root meaning "to cover."

It forms all or part of: aperitif; apertive; aperture; barbican; cover; covert; curfew; discover; garage; garment; garnish; garret; garrison; guarantee; guaranty; kerchief; landwehr; operculum; overt; overture; pert; warn; warrant; warrantee; warranty; warren; wat; Wehrmacht; weir.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vatah "enclosure," vrnoti "covers, wraps, shuts;" Lithuanian užveriu, užverti "to shut, to close;" Old Persian *pari-varaka "protective;" Latin (op)erire "to cover," (ap)erire "open, uncover" (with ap- "off, away"); Old Church Slavonic vora "sealed, closed," vreti "shut;" Old Irish feronn "field," properly "enclosed land;" Old English wer "dam, fence, enclosure," German Wehr "defense, protection," Gothic warjan "to defend, protect."

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warrantee (n.)
"person to whom a warranty is given," 1706, from warrant (v.) + -ee.
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guaranty (n.)
"act or fact of guaranteeing, a being answerable for the obligations of another," 1590s, garrantye, from earlier garant "warrant that the title to a property is true" (see guarantee (n.)), with influence from Old French garantie "protection, defense; safeguard, warranty," originally past participle of garantir "to protect," from the same source. The sense of "pledge given as security" that developed 17c. in guarantee might reasonably have left the sense "act of guaranteeing" to this form of the word, but the forms remain confused.
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guarantee (n.)

1670s, "person that gives security," altered (perhaps via Spanish garante or confusion with legalese ending -ee), from earlier garrant "warrant that the title to a property is true" (early 15c.), from Old French garant "defender, protector; warranty; pledge; justifying evidence," from a Germanic source, from Proto-Germanic *war- "to warn, guard, protect," from PIE root *wer- (4) "to cover." For form evolution, see gu-. Sense of the "pledge" itself (which is properly a guaranty) developed 18c.

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