Scout's honour
An oath that one is being ingenuous or honest, or will uphold a promise or duty. Alludes to the oath taken by a member of the Scouting movement to be upstanding, trustworthy, and honest. Primarily heard in UK. I swear that I'll behave myself at your brother's wedding, Scout's honour! A: "Are you really telling me the truth about what happened to my car?" B: "Scout's honour!"