1swear 
/ˈsweɚ/ verb
swears; swore /ˈsweɚ/ verb /ˈswoɚ/; sworn /ˈswoɚn/; swear·ing 1 a : to state (something) very strongly and sincerely [+ obj] ▪ I swear (that) I was there. [=I was definitely there; I am telling you the truth when I say that I was there] ▪ Her English is so good that you would swear she has been studying it for years. [=you would really think that she has been studying it for years, even though she has not] ▪ You'd swear (that) the jewels are real. [=the jewels look very real even though they are not] ▪ I could have sworn that I left my keys on the counter. [=I'm very surprised that my keys are not on the counter because I definitely remember putting them there] ▪ (US) She swore up and down [=insisted] (that) she didn't do it. ▪ (Brit) He swore blind [=insisted] (that) he was innocent. [no obj] ▪ I didn't do anything wrong. I swear (on my mother's grave). [=I am being absolutely honest when I say that I didn't do anything wrong] ▪ I wouldn't swear to it [=I'm not absolutely sure about it], but I think he's written three books so far.
b : to promise very strongly and sincerely to do or not do something [+ obj] ▪ He swore [=vowed] to seek revenge. ▪ He swore revenge on the killers. ▪ I swear (that) I'll never do that again. [no obj] ▪ I swear to God, I'll kill him if he comes back. ▪ I'll never do that again. Swear to God. [=I promise you that I'll never do that again] 2 [no obj] : to use offensive words when you speak
▪ Don't swear in front of the children. — often + at ▪ The other driver swore at me and drove away. 3 : to make a formal or official promise especially in a court of law [+ obj] ▪ I do solemnly swear to tell the whole truth. ▪ They swore (an oath of) allegiance to the United States of America. [=they formally promised that they would be loyal to the United States of America] [no obj] ▪ Witnesses are required to swear on the Bible [=to put a hand on the Bible and make a formal promise to tell the truth] before they testify. 4 [+ obj] : to cause (someone) to make a promise
▪ He swore us to secrecy/silence. [=he made us promise not to tell anyone his secret] — often used as (be) sworn ▪ Witnesses are sworn to tell the truth.
b : to promise very strongly and sincerely to do or not do something [+ obj] ▪ He swore [=vowed] to seek revenge. ▪ He swore revenge on the killers. ▪ I swear (that) I'll never do that again. [no obj] ▪ I swear to God, I'll kill him if he comes back. ▪ I'll never do that again. Swear to God. [=I promise you that I'll never do that again]
▪ Don't swear in front of the children. — often + at ▪ The other driver swore at me and drove away.
▪ He swore us to secrecy/silence. [=he made us promise not to tell anyone his secret] — often used as (be) sworn ▪ Witnesses are sworn to tell the truth.
I swear
informal — used for emphasis ▪ I swear, every time I see her she's got a new boyfriend. ▪ I swear, if I had a nickel for every time you did that, I'd be rich.
swear by [phrasal verb]
swear by (something) : to have or express a lot of confidence in (something) : to say that (something) is very good or useful
▪ She swears by this diet. ▪ His method sounds pretty strange, but he swears by it.
▪ She swears by this diet. ▪ His method sounds pretty strange, but he swears by it.
swear in [phrasal verb]
swear (someone) in or swear in (someone) : to place (someone) in a new office or position by having an official ceremony in which that person makes a formal promise to do the work properly, to be honest and loyal, etc.
▪ When do they swear him in? — often used as (be) sworn in ▪ The new president will be sworn in tomorrow. — see also swearing-in
▪ When do they swear him in? — often used as (be) sworn in ▪ The new president will be sworn in tomorrow. — see also swearing-in
swear off [phrasal verb]
swear off (something) chiefly US : to stop doing, having, or being involved in (something)
▪ He was so disgusted by the election results that he swore off politics for good. ▪ She tried to swear off chocolate, but she couldn't do it.
▪ He was so disgusted by the election results that he swore off politics for good. ▪ She tried to swear off chocolate, but she couldn't do it.
— see also sworn
