Definition & Meaning of "by gum"

What is the origin of the idiom "by gum" and when to use it?

The idiom "by gum" is used to express mild surprise, emphasis, or affirmation. It is commonly used in informal speech, especially in British and Northern English dialects. The origin of this phrase is not precisely documented, but it has been in use for many years as a way to add emphasis or to express surprise or determination.