Old English swerian "take an oath" (class VI strong verb; past tense swor, past participle sworen), from Proto-Germanic *swērjanan (source also of Old Saxon swerian, Old Frisian swera, Old Norse sverja, Danish sverge, Middle Dutch swaren, Old High German swerien, German schwören, Gothic swaren "to swear"), of uncertain origin, perhaps from a PIE *swer- "to speak, talk, say" (source also of Old Church Slavonic svara "quarrel," Oscan sverrunei "to the speaker").
Also related to the second element in answer. The secondary sense of "use bad language" (early 15c.) probably developed from the notion of "invoke sacred names." Swear off "desist as with a vow" is from 1898. Swear in "install (someone) in office by administration of an oath" is attested from 1700 in modern use, echoing Old English.
[Swearing and cursing] are entirely different things : the first is invoking the witness of a Spirit to an assertion you wish to make ; the second is invoking the assistance of a Spirit, in a mischief you wish to inflict. When ill-educated and ill-tempered people clamorously confuse the two invocations, they are not, in reality, either cursing or swearing ; but merely vomiting empty words indecently. True swearing and cursing must always be distinct and solemn .... [Ruskin, "Fors Clavigera"]
Middle English cursen, from Old English cursian, "to wish evil to; to excommunicate," from the source of curse (n.). Intransitive meaning "swear profanely, use blasphemous or profane language" is from early 13c. (compare swear (v.)). The sense of "blight with malignant evils" is from 1590s. Related: Cursed; cursing.
Old English answarian "make a statement in reply," from and- "against" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before") + swerian "to swear" (see swear), suggesting an original sense of "make a sworn statement rebutting a charge."
Meanings "conform, correspond" and "be responsible for" are from early 13c; that of "suffer the consequences of" is from late 13c.; that of "respond in antiphony" is from early 15c. Sense of "respond in act or action, give back in kind" is from 1570s; that of "solve, find the result of" is from 1742. Related: Answered; answering. The telephone answering machine so called from 1961.
Old English andswaru "a response, a reply to a question," from and- "against" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before") + -swaru "affirmation," from swerian "to swear" (see swear), suggesting an original sense of "sworn statement rebutting a charge." Meaning "solution of a problem" is from c. 1300.
It is remarkable that the Latin expression for answer is formed in exactly the same way from a verb spondere, signifying to engage for, to assure. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]
A common Germanic compound (cognates: Old Saxon antswor, Old Norse andsvar, Old Frisian ondser, Danish and Swedish ansvar), implying a Proto-Germanic *andswara-. The simpler idea of "a word in reply" is expressed in Gothic anda-vaurd, German Antwort.
mid-15c. "swear falsely" (implied in perjured; late 13c. in Anglo-French), from Old French parjurer "to break one's word, renege on a promise" (11c.), from Latin periurare "to swear falsely, break one's oath," from per "away, entirely" (see per) + iurare "to swear" (see jury (n.)). Reflexive sense, "make (oneself) guilty by testifying falsely" is from 18c.