A two-word swear that substitutes for a more offensive two-word swear that ends in damn.
I hurt my dad gum finger opening that jar of pigs feet.
Modification of the term god damn. Comes from the midwest, as near as I can tell - my grandfather (an Illinois native) used to say it a lot.
Note the term is always used as an adjective, and NEVER as a verb. So you can have (day) a dad gum toll booth attendant, but you don't dad gum the toll booth attendant. The only exception to this rule is that "Dad gummit" is a suitable substitute for "goddammit"
Dad gummit! Where is the dad gum screwdriver?