Synonyms for Swear:

n.

abuse. attest, undertake, vouch, vouchsafe, warrant, swear by, give (someone) your word. justify. swear (noun)
affirm, assert, aver, avow, bank, blaspheme, curse, cuss, depone, depose, imprecate, rely, swan, trust, verify.

v.

cuss (verb)
blaspheme, curse, cuss, nag, scold.
declare under oath (verb)
affirm, assert, attest, avow, depose, promise, trust, vouch, vow, warrant, swear by.
promise (verb)
agree, assure, consent, guarantee, pledge, promise, stipulate, vow.
speak profanely; be vulgar (verb)
blaspheme, curse, cuss, imprecate.

Other synonyms:

undertake. attest, warrant. abuse. assert
assert, vouch.
promise
assure.
Other relevant words:
abuse, affirm, assert, attest, aver, avow, bank, depone, depose, imprecate, justify, rely, swan, trust, undertake, verify, vouch, vouchsafe, warrant, swear by.

Usage examples for swear

  1. Your looks'll swear it's the truth. – Lonesome Land by B. M. Bower
  2. I don't know into what political combinations Barroux may have entered, but I am ready to swear that he put nothing in his pocket, for he is the most honest of men. – Paris From the "Three Cities" by Emile Zola
  3. So you think that I would swear to a lie, Philip? – Mr. Marx's Secret by E. Phillips Oppenheim