Middle English sweren, from Old English swerian, swerigean, "take or utter an oath, make a solemn declaration with an appeal to divinity" (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"). This is of uncertain origin. The old explanation (Pokorny, Watkins) has it from a PIE *swer- "to speak, talk, say" (source also of Old Church Slavonic svara "quarrel," Oscan sverrunei "to the speaker"). Boutkan suspects a substratum word, or, if it is IE, writes that a connection to Latin verbum "seems more promising." It is related to the second element in answer. A Middle English nounswaremeant "an answer, a reply; speech, utterance," from Old English-swaru, and from the Old Norse cognate. The secondary sense of "use profane language" (early 15c.) probably developed from the notio...
