There are plenty of ways to say I understand. Umble-cum-stumble is the one that sounds least like it.

J. Redding Ware put it in Passing English of the Victorian Era (1909) under the label Low. Class — the speech of working people — and defined it in two words: Thoroughly understood. Not half-grasped, not more or less. Completely.

What makes it odd is that the sound and the meaning point in opposite directions. Stumble is what you do when you have not got the hang of something, and umble sounds like mumble or fumble. Yet the phrase means the message has gone in perfectly.

Ware gives no explanation and no example, and we have found no source that does. What can be said is that it is built the way a lot of English slang is built: two rhyming nonsense-ish words hitched together, here with the Latin cum — with — in the middle, the same cum found in kitchen-cum-dining-room. It is the kind of jingle that sticks because it is fun to say, whatever it once meant.

Green's Dictionary of Slang lists it as a verb, to understand comprehensively, citing Ware and nothing else. So Ware's entry is the entire record.

As a word it has one obvious use: the cheerful reply when someone has finished explaining something complicated. Umble-cum-stumble — got it, all of it, no need to go over it again.