The root word join is the basis of a small group of words with some sense pertaining to unity, though many originally had a legal connotation (and some still do). Here are those words and their definitions.
Join itself stems from the Latin word iungere, meaning “join together,” “unite,” or “yoke.” (In Latin, i could be pronounced as a vowel or a consonant; the latter sound was equivalent to y, which came to be pronounced like j in English.)
Joint originally referred to a part of the body where two bones meet; this sense was later extended to any connecting point and to a cut of meat. Joint, as a slang term for a marijuana cigarette, might derive from the fact that it is often shared, but alternatively, it may be borrowed from earlier use of the term to describe a drug syringe, though the origin of that usage is obscure.
The meaning of joint as a physical location where people met, initially in the sense of a disreputable establishment, probably derived from the idea of a smaller chamber adjoining a main room, where secret meetings, perhaps involving illegal activity, could be held. (This illicit sense probably inspired the use of joint as slang for jail or prison.) The adjectival sense of joint, meaning “sharing” or “united,” developed from the noun.
Something disjointed lacks order and organization; the verb form disjoint is obsolete except in the mathematical sense of having no elements in common.
A joiner is a carpenter who specializes in intricate woodworking, often involving joining pieces of wood to create boxes or furniture. The word also denotes someone with a proclivity for becoming a member of clubs or other organizations.
Joinder is a word essentially confined to legal usage to refer to an act of joining together; it is also a rarely used synonym for the grammatical term conjunction. However, rejoinder, originally referring to a defendant’s answer to a charge, acquired a broader sense of a response, with the connotation of an angry reply to a critical comment. (Rejoin itself, which now means “reunite,” originally connoted a response in legal proceedings.)
The similar-sounding jointure refers to a joint or an act of joining, although it is mostly used in the legal sense of settlement of an estate.
Adjoin originally meant “ally” or “unite,” but the later sense of “be adjacent to” became predominant; the adjectival form is adjoining. To enjoin, usually used in a legal context, is to require or prohibit. Conjoin, meaning “come together,” is most frequently seen in its adjectival form in the phrase “conjoined twins,” referring to twins whose bodies are partially combined; conjoint is the basic adjectival form. Subjoin, meanwhile, means “add” or “append.”