O.E.
swerian "take an oath" (class VI strong verb; past tense
swor, pp.
sworen), from P.Gmc.
*swarjanan, from root
*swar- (cf. O.S.
swerian, O.N.
sverja, Dan.
sverge, O.Fris.
swera, M.Du.
swaren, O.H.G.
swerien, Ger.
schwören, Goth.
swaren "to swear"), from PIE base
*swer- "to speak, say" (cf. O.C.S.
svara "quarrel"). Also related to the second element in
answer. The secondary sense of "use bad language" (c.1430) developed from the notion of "invoke sacred names."
Swear-word is Amer.Eng. colloquial from 1883.
Swear off "desist as with a vow" is from 1898.