Mother lode is the correct spelling of the phrase meaning an abundant or rich source. It was originally an American mining term, referring to the main vein of ore in an area, but it’s now most often used in the figurative sense. Mother load, a common misspelling, sort of makes sense, but it’s not the conventional term, and many readers will view it as incorrect.
Mother lode is usually two unhyphenated words, although a few publications spell it motherlode. Both are more common than the hyphenated mother-lode.
Examples
The misspelled mother load appears about a tenth as often as the correct term. For example, these writers might want to be more careful:
The Teke family hit the mother load when it comes to helping another family with a precious gift. [WKRC Cincinnati]
These past few weeks have delivered a proverbial mother load of video game comics. [Game Focus]
And these writers use mother lode correctly:
He hit the mother lode, but not once did Josh Ferrin even think of laying claim on the more than $45,000 cash that he found in his garage. [Deseret News]
It is the newest find in a mother lode of prehistoric bones uncovered during a reservoir expansion project last fall … [Real Vail]
But there’s a mother lode of original Mother’s Day gifts to be had, many at local places. [Seattle Times]
