Is it ensure, insure, or assure?
early 14c., momelen, "to eat in a slow, ineffective manner" (perhaps "to talk with one's mouth full"), probably frequentative of interjection mum. The -b- is excrescent. Meaning "to speak indistinctly" is from mid-14c. Related: Mumbled; mumbling.
1902, from mumble (v.).
1. Said when the correct response is too complicated to enunciate, or the speaker has not thought it out. Often prefaces a longer answer, or indicates a general reluctance to get into a long discussion. "Don't you think that we could improve LISP performance by using a hybrid reference-count transaction garbage collector, if the cache is big enough and there are some extra cache bits for the microcode to use?" "Well, mumble ... I'll have to think about it."
2. Yet another metasyntactic variable, like foo.
3. Sometimes used in "public" contexts on-line as a placefiller for things one is barred from giving details about. For example, a poster with pre-released hardware in his machine might say "Yup, my machine now has an extra 16M of memory, thanks to the card I'm testing for Mumbleco."
4. A conversational wild card used to designate something one doesn't want to bother spelling out, but which can be glarked from context. Compare blurgle.
5. [XEROX PARC] A colloquialism used to suggest that further discussion would be fruitless.