The air is thick with humidity and allergens, and I am peevish.
Listen to me, O People, when I say unto you, the phrase is not “mother load”, it is “mother lode.”
The term comes from mining, specifically gold and silver mining, where it refers to a principle vein or group of veins of ore. The Mother Lode, in the United States, is an area of hard-rock gold deposits in California’s Sierra Nevada, running through a zone 120 miles long and in some places almost 4 miles wide. (It was, unsurprisingly, discovered during the California Gold Rush.)
A mother lode of something, then, is an abundant source or principle supply of that thing. The “mother” part comes from the use of “mother” to refer to a source or origin: “Mother of pearl” refers to the substance known as nacre, with which a mollusk encases the bit of irritant which forms the center of a pearl; “mother of vinegar” is the naturally-occurring bacterial culture which, when added to wine or other substances, causes them to ferment into vinegar; and “mother” or “mother dough” is a term sometimes used in baking to refer to a naturally-cultivated yeast starter.
And the “lode” part? That’s from the Old English lād, meaning “a way” or “a course” – usually a watercourse of some sort. So a lode is a way or course that ore runs through, like water runs in a stream.