kunmeti

Decode it: kun plus meti. Together plus to put. To put together. There is no idiom here and nothing has drifted: kunmeti means precisely what its two pieces mean, and it means it in both the concrete sense (assemble the shelf) and the abstract one (put an argument together). A preposition used as a prefix is one of Esperanto's oldest and most productive habits, and this is the cleanest example of it in the chapter.

Now the good part: kunmetitaj vortoj. The past passive participle of this verb is the standard grammatical term for a compound word. Fulmotondro is a kunmetita vorto; so is taglibro, vortaro and sunsubiro. The operation of making them is kunmetado, and Rule 11 of the Fundamento is the rule that licenses it: compound words are formed by simply writing the roots one after the other, with the main word at the end. That is the whole rule. Nothing about linking vowels is mandatory, nothing about which roots may combine, no list of permitted compounds. The compounding article works through what falls out of that, including when to insert the optional -o- for the sake of your reader's tongue.

So the verb describes the language's own method. It is worth pausing on that, because it is unusual. Most languages have a technical word for compounding that is opaque to their own speakers: English has compound, from Latin, and an English speaker cannot see the operation inside it. In Esperanto the word for putting words together is the word for putting anything together, and a beginner meets it in the sentence "I assembled the furniture" long before they meet it in a grammar. The word-building article uses it in both senses on the same page without any awkwardness.

Direction and neighbours. Kunmetiĝi is to come together of itself; dismeti is to take apart, using dis- for scattering; kunigi is to unite two things, which is broader and more abstract than physically putting them together; kunveni is to meet, which is people rather than parts. And kunmetaĵo is a compound as a thing, chemical or grammatical. The prefix kun- also gives you kunlabori (to collaborate), kunsenti (to sympathise) and kunporti (to bring along), all of which you can now read without being told.