By the Cambridge Dictionary
To legally forbid or stop something by order of a court
Enjoin also means to order or strongly encourage someone to do something
By the Oxford Dictionary
- Instruct or urge (someone) to do something
- Law Prohibit someone from performing by issuing an injunction
So basically, "He is enjoined to stop" can mean either way.
Enjoin: : the common usage is from the old French "enjoindre", impose:
- [often passive] enjoin somebody to do something enjoin something (formal) to order or strongly advise someone to do something; to say that a particular action or quality is necessary.
The other meaning is only of legal usage and is related to an injunction:
To direct, require, command, or admonish.
Enjoin connotes a degree of urgency, as when a court enjoins one party in a lawsuit by ordering the person to do, or refrain from doing, something to prevent permanent loss to the other party or parties. This type of order is known as an Injunction.
enjoin somebody from doing something (law) to legally prevent someone from doing something, for example with an injunction (= official order)
Enjoin (v.): etymology:.
- c. 1200, engoinen, "to prescribe, impose" (penance, etc.), from stem of Old French enjoindre (12c.) "impose (on), inflict; subject to; assign (to)," from Latin iniungere "to join, fasten, attach;" figuratively "to inflict, to attack, impose," from in- "on" (see in- (2)) + iungere "to join" (see jugular).
I do not have a well-documented answer, so please take careful note that this is purely speculative.
The etymology of the first definition of enjoin, where you order someone to do something, is fairly straightforward and well-documented.
http://www.etymonline.com/index.php?term=enjoin
c. 1200, engoinen, "to prescribe, impose" (penance, etc.), from stem of Old French enjoindre (12c.) "impose (on), inflict; subject to; assign (to)," from Latin iniungere "to join, fasten, attach;" figuratively "to inflict, to attack, impose," from in- "on" (see in- (2)) + iungere "to join"
So the question is, how did it also become used to mean "to prohibit"?
Well, as noted Merriam Webster and other dictionaries, this meaning primarily refers to court orders. In this sense, "enjoin" and "injunction" are similar-- the courts enjoin you to do something or to refrain from something by issuing an injunction, and both words share the same root.
It is really just a game of semantics to determine whether an injunction is an order to do something or an order not to do something. For example, if a judge orders you to immediately stop using a trademarked logo, is he ordering you to stop doing something, or to start doing something? You'll have to immediately take action to cover/hide the logo from any storefronts or products you sell, alter your web page design, and so on.
Injunctions are very often orders to "not do" something, so it makes sense that it would pick up this connotation. If someone says "I got an injunction" it tends to conjure up the idea that they got a legal order for you to stop doing something immediately.
And any time an injunction is issued, you've been enjoined. Enjoined to do what? Well, very often, to stop doing what you're doing.
As one comment points out (eventually -- you actually have to follow the link and read), enjoin is an example of a contronym. To quote Kimberly Joki in Grammarly:
A contronym, often referred to as a Janus word or auto-antonym, is a word that evokes contradictory or reverse meanings depending on the context. Specifically, a contronym is a word with a homonym (another word with the same spelling but different meaning) that is also an antonym (a word with the opposite meaning).
The article lists 10 verbs that, like enjoin, are contronyms.
