Yes, I’m doing the word ply this week. I like what I did with the title, because not only is plyers a homophone for pliers, a real word, the suffix -er means a person/thing that has to do with something, like laborer to labor. Get it? Funny, right? Hello? Guys?
Ply has three main definitions, work with or at (ply a trade), bend or fold, and a layer of something. The first two showed up in the late fourteenth century are closely related, although in kind of a weird way. “Work with” ply is actually short for another word, applien, which was really used in English at one time. And yes, it sounds a lot like apply, but applien doesn’t come from apply, except in the sense that they both come from the classical Latin plicare, fold, and also where the other ply comes from. The third ply showed up in the mid sixteenth century, coming from the Middle French pli, a fold, and Old French ploi, layer. It also comes from plicare because of that whole fold/bend thing.
Note that this is not something that seems like apples. That would be appley. Apply showed up in the late fourteenth century, coming from the Old French aploiier, same meaning, and the classical Latin applicare, to connect. Plicare, fold, is the root word here, with the prefix a- (or ad-), meaning to or towards. Apply has a lot of different meanings today. You apply for a job. You apply ointment to the affected area. But originally, it meant to put yourself at work towards a task, and a figurative definition of being in contact (i.e. ointment to skin). Interestingly enough, job apply only showed up in the eighteen fifties, although it’s quite similar to the original definition of apply. Well, I think it’s interesting.
Showed up in the late fourteenth century, where it meant enfold or entangle. Seriously. It comes from the Old French emplier and classical Latin implicare, involve. The enfold definition makes sense since plicare means fold and in- means, well, in. Like apply, it just went off in a completely new direction. Latin meant enfolding in the figurative sense, so enfolding in an event (or whatever) would be involving. English kind of took it from there.
Reply showed up in the late fourteenth century with the same definition. It comes from the Old French replier, Late Latin replicare. The re- prefix means back, and with plicare, to fold, it means to fold back again. Like all the other words here, its meaning comes from the figurative use of the word.
Comply showed up in the early fourteenth century, where it meant fulfill or carry out, like one would an order (at least getting to the definition of agreement makes sense from there). Comply was compli in Old French (same definition) and in Vulgar Latin it was complire and classical Latin complere. Notice there’s not a plicare in there? That’s because although comply may have been influenced by ply, it actually comes from complete. Despite what it looks like, it’s not a ply word! It’s actually made up by the prefix com-, with or together, and plere, fulfill.
Supply just happens to be in the same boat as comply. It’s not from plicare but plere, being a combination of sub- (from below) and plere, fulfill. To fulfill from below. I’m going to guess that’s figurative. Oh, and it showed up in the fourteenth century.
TL;DR: There are two origin words for -ply words because we dropped the c from one of them.