A manifold is a wide and/or bigger pipe, or channel, into which smaller pipes or channels lead.
The Dutch translation for this word is spruitstuk. This word sounds like total gibberish (unless you are a car mechanic or engineer, I guess). I assume that non-mathematicians think exactly the same when we use words like sheaf, blow-up or formality.
What makes this even funnier to me is the following story. When I was a bachelor student I had a course on representation theory whose translated title was (or should've been) "Finite-dimensional algebras". In Dutch this would be: "Eindigdimensionale algebra's". Dutch has some specific rules on joining words, as in German. So when the title of the course was misspelt as "Eindig dimensionale algebra's" someone interpreted this typo in the wrong way and corrected it to "Eindige dimensionale algebra's", meaning "Finite and dimensional algebras". I never figured out what a dimensional algebra is, and it makes me wonder when the first mathematics course on spruitstukken will be taught.
And congratulations to Ian Agol for winning this year's (or next year's?) Breakthrough Prize! It is because of this NY Times article about it that I learned about manifolds in fluid mechanics.