A lot of people swear their lives on it, while others can't stand it.

A lot of people swear their lives for it, while others can't stand it.

What's the difference between on and for in these sentences?

  • 1
    What are your sources for these phrases? I have never heard " A lot of people swear their lives for it" before. More context might help. – user3169 Jan 24 '15 at 1:55
  • I was on irc taking a chit chat with some americans, and one of them came out saying the second phrase. I thought it meant that people would give their lives for it. – Hachimana Jan 24 '15 at 2:17
  • 1
    Almost nobody still says "I swear my life on it" ("it" being some prior assertion the speaker made; he's effectively saying "May God strike me dead if what I said is untrue"). Nobody ever swears "on" anything except assertions or Bibles. And I've never heard of anybody swearing their life for some thing - just feasibly you might swear your life for a loved one (or might have, a century or two ago), but that would have meant promise to dedicate the remainder of your life to the wellbeing of the other person. – FumbleFingers Jan 24 '15 at 2:24

A lot of people swear their lives on it.

A lot of people swear their lives for it.

Neither of these sentences seems idiomatic. Nevertheless, we have the following phrasal verbs in use.

  • Swear on one's life/honor used for emphasizing strongly that you will do something or what you are telling is true (McMillan). For example, I swear on my life I will always remain faithful to you. Do you swear on you honor that you will never tell anyone (Longman).

  • Swear for used for giving assurance or to guarantee such as I swear for his safety.


As FumbleFingers said in the comment, if you swear on something, you are promising something. E.g., "I swear on a stack of Bibles, I am telling the truth," or "I swear upon my honor that I will pay you back tomorrow." However, "I swear my life on it" is a very unusual formulation. While it will probably be taken as "May I be struck dead (by a higher power) if I am lying," I would avoid using it. (Instead, consider, "I would stake my life on it.")

If you swear your life for something, it is still a somewhat odd construction, but would probably be taken to mean that you have taken an oath to defend, or fight for, someone, some country, or some cause, even if it means you die in doing so. It would seem archaic, like a knight swearing their life for the cause of their king. It would be more common to see "pledge" or "promise" used instead of "swear." (E.g., "I pledge my life and my sacred honor to the cause.")

  • So should I use "on" in every single situation? – Hachimana Jan 25 '15 at 23:45
  • It's probably a good default, yeah. Just make sure that when you use "swear on" that you are swearing on something -- metaphoric or literal. As noted above, neither of the sentences that you have in the example looks quite right. – A.Beth Jan 26 '15 at 1:29

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