A monkey is a simian primate closely related to a human being, but, and this is important, not as closely related to a human being as the higher apes.
There are currently 264 discrete species of monkey on Roundworld. As the Librarian would be the first to tell you, there are only seventeen species of ape. (It is to be hoped that these numbers have not gone down by the time you finish reading this article. )
Therefore the Apes represent a smaller and more select family within the animal kingdom (as the Librarian would also be at pains to point out)
The m-word is not one you use anywhere near the Librarian, who has fixed ideas about its use and application, eg to him. Only three people in the history of the Chronicles have escaped the usual painful fate for applying the wrong descriptive term to the Librarian. This shows that he has a certain discretion in these matters and is prepared to overlook a genuine mistake, especially if it's somebody he likes (Agnes Nitt), or somebody too slow-witted to know better and who has been put up to it by his mates as a practical joke (Carter the baker). But he will get the person sniggering in the background who thinks he's safe and can watch the joke unfold with impunity. As Weaver the thatcher found out, when he found himself being thrown over the Lancre bridge by a mystery assailant later...
Albert appears to get away with calling the Librarian a monkey ("A monkey? In my university?") at first, but the ape later exacts his revenge.
In The Last Continent, one of the wizards accidentally calls the Librarian a monkey and escapes without penalty, leading the wizards to realize just how sick the Librarian actually is.
And the supreme example of use of the M-word occurs in The Last Hero, where the intrepid crew of the Kite reach Cori Celesti and are debating with the Gods. Just for a second or two, Blind Io himself is in danger of being on the receiving end of correction from the Librarian after suggesting that his spiritual needs could be better taken care of by an interview with the Monkey God. Only Rincewind's quick thinking saves there being a serious upset and another duffing-over for the God in his own throne-room (the first having been administered by Om.) We learn that the Librarian's wrath can be assuaged by the provision of three thousand file cards, a new stamp, five gallons of ink and a red balloon.
Anyone else using the m-word out of malice or willful ignorance is dealt with without mercy and shown an elementary biology lesson - ie, can a mere monkey hold somebody upside down by their ankles and bounce their head repeatedly off a hard unyielding floor?
In an even more elementary biology lesson, Ponder Stibbons in The Last Continent recalls how, as a small child, his nurse had told him that monkeys were bad little boys who hadn't come in when called. This didn't exactly give him the idea of evolution, but he later had to admit that she might have been onto something...
Nobby Nobbs has occasionally been mistaken for a pet monkey, albeit probably only on his better days.