'Bespoke' is one of those words that, when I read it, I know I don't understand but pass over as if I did. In my mind bespoke always means something like 'spoken for', so when I read it in a sentence like 'He was wearing a bespoke suit,' as I did in some crime story or other recently, my mind does a weird swerve and thinks of a 'spoken for' suit, which doesn't make a whole lot of sense. But as that doesn't usually hinder the general sense of the story--and also doesn't come up all that often-- I've let it slide, as frankly I often do with unfamiliar descriptions of fashion.
Okay, now that I know what a bespoke suit is, I realize that what I wrote above isn't totally true. When I read this kind of word, I actually fill in some sort of image. Because in whatever that story was I was reading, the suit I filled in was a kind of blackish one with gold pinstripes, and there was some sense of it being Sunday go to meeting best, as worn by a country person who didn't always wear suits. I saw a little hay in there somewhere, maybe in the guy's mouth. Probably got the word 'spokes' mixed into this in my head.
The idea of a bespoke suit comes from the word 'bespeak', of course, in the sense of 'to speak for something'--which fits my idea of it, but has the further specialized sense here of 'to give order for it to be made'.
This comes about, I think, from the very loose sense of 'bespeak' itself, at least according to the OED. 'Bespeak' comes from the Old English besprecan which came from a common German compound, meaning 'to call out'. In English it took up a variety of very meanings, which the Online Etymology dictionary has arising quite independently of each other, including 'speak up', 'oppose', 'arrange' 'request' and 'order goods'.
The reason for this is that the 'be-' prefix has quite an extraordinary range, and can be an intensifer, a privative (meaning deprive of, such as a beheading), a causative, which is a word that shows something has transformed something else (I would again think behead, but I digress.) In short, it can have 'just about any sense required'. Be- was useful in forming many new words of the 16th and 17th centuries, more than a few of which have since disappeared from view. The online etymology gives examples of two nice ones: 'bethwack', which is obvious, and 'betongue', 'to assail in speech, or scold', which is not.
Speaking of bespoke suits, here's a good blog post about Rufus Sewell recklessly endangering one.