Learn English – What does the fox say

You seem to be asking about the origin of the term as used in category theory. The history of the term there is somewhat unclear, but it can at least be traced back a little ways:

• The term is sometimes attributed to Mac Lane, but this seems to be inaccurate; however, the widespread use of the term is probably due to his influential "Categories for the Working Mathematician", replacing the remarkably terrible term "triple".

  The frequent but unfortunate use of the word "triple" in this sense has achieved a maximum of needless confusion, what with the conflict with ordered triple, plus the use of associated terms such as "triple derived functors" for functors which are not three times derived from anything in the world. Hence the term monad.

• Mac Lane's use of the term was apparently prompted by J. P. May:

  The name "operad" is a word that I coined myself, spending a week thinking about nothing else. Besides having a nice ring to it, the name is meant to bring to mind both operations and monads. Incidentally, I persuaded MacLane to discard the term "triple" in favor of "monad" in his book "Categories for the working mathematician", which was being written about the same time. I was convinced that the notion of an operad was an important one, and I wanted the names to mesh.

• Elsewhere, Ross Street attributes the term to Jean Bénabou:

  Meanwhile Jean Bénabou had invented weak 2-categories, calling them bicategories. (...) He pointed out that a lax functor from the terminal category 1 to Cat was a category A equipped with a "standard construction" or "triple" (that is, a monoid in the monoidal category [A, A] of endofunctors of A where the tensor product is composition); he introduced the term monad for this concept.

  The attribution to Bénabou is also mentioned here.

• The motivation for the term is to suggest a relationship with monoids, as can be deduced from the construction given in the quote above, and the Greek root "monos" comes second-hand. The connection to philosophy in general, or Leibniz in particular, is often asserted but never to my knowledge supported in any way. More likely if anything would be a connection to the term "monad" used in non-standard analysis, also related to Leibniz, but I'm not sure what the conceptual link there would be. An anecdote from Michael Barr relates the first use of the term:

  (...) The attendance consisted of practically everyone in the world who had any interest in categories, with the notable exception of Charles Ehresmann. (...) One day at lunch or dinner I happened to be sitting next to Jean Benabou and he turned to me and said something like "How about 'monad'?" I thought about and said it sounded pretty good to me. (Yes, I did.) So Jean proposed it to the general audience and there was general agreement.

  The off-the-cuff nature of the suggestion, and immediate positive response from a large audience, suggests that there's probably no written record of the term being introduced formally. It's certainly possible that the word was borrowed from use in philosophy or elsewhere, but in any case there appears to be no connection more meaningful than the level of "cheap pun".

As far as I know, the only way you're going to get a better answer than that is by asking Bénabou himself.

I don't like querier/queryer/queryist/querist at all. A century or two ago, querist was actually quite common - but it's massively declined since then, and sounds seriously "Victorian" to me.

I'd go with querent (also sometimes spelled querant) as the more "modern" usage...

EDIT: Also note from that chart (click on it to see the complete page which I can't reproduce here) that the forms queryer, queryist (spellings which were never common) are now both so rare they can't even be charted for the past half-century.