Is there a term for co-opting one word into a secondary, derogatory meaning?

Solution 1:

I think you are referring to the phenomenon of semantic drift, a change in meaning of a word over the course of time.

In the particular examples you describe the semantic change would take the form of semantic broadening or generalization. This is of course the commonest type of change. So your blank could be filled with broadened or generalized.

There is a wikipedia article which lists the various types of semantic change. In a more convenient form I think the changes can be considered under the following categories:

Generalization

Narrowing

Degeneration/Pejoration

Amelioration/Elevation

Hyperbole

Metaphor

Metonymy

Synechdoche

Solution 2:

Of all of your examples, the word retarded is really the only one to have suffered the fate that's the gist of this sentence of yours:

OCD originally referred to a serious anxiety disorder but was [word] into a description roughly meaning "appreciates order and organization."

What's happened to it is termed:

pejoration

1) The process or condition of worsening or degenerating.

2) Linguistics The process by which the meaning of a word becomes negative or less elevated over a period of time, as silly, which formerly meant "deserving sympathy, helpless or simple," has come to mean "showing a lack of good sense, frivolous."

OED features the verb too:

pejorate, v.tr.

To make worse, deteriorate, worsen.

OED's is the general sense of the verb, but you can work out the more specific, linguistic one, as well as feel free to derive the passive participle: pejorated. An examplar:

sanctimonious (adj.), sanctimony (n.) In the mid-sixteenth century the adjective meant "holy, saintly," but by the early seventeenth century it had come to mean both "holy" and "hypocritically holy," and today it has pejorated to mean only "affectedly saintly, falsely holy."

Regarding your other examples—OCD, ADD, dyslexia—the usage you're so peeved by is nothing but a hyperbole. I reach for a hyperbole at least a couple dozen times a day. (There comes another one). OCD, I guess. (And another one.) In such use OCD doesn't lose its meaning; it's just used in a different context. Put differently, your sentence is a false assertion. To my mind.

Nonetheless, sans the infelicitous examples, the question you've posed is perfectly suitable for ELU. Here are some more words (participles) to express the general idea of rendering something less worthy:

reduced

demoted

disrated

declassed

bastardized

profaned

vulgarized

prostituted (Motion-picture craftsmen shouldn't be prostituted into making junk and trash. / Professor Clayton Christensen at Harvard Business School summarized my own frustrating experiences when he said, “There’s usually some process by which a potentially great idea gets prostituted into something lacklustre”.)

perverted

vitiated

defiled (The Mortgage Arrears Resolution Process, lauded as a consumer protection device, has been defiled into a process designed to give banks intrusive and powerful control over Irish mortgage holders.)

degenerated

scuzzed up (Slang.)

I'm sure many more can be thought of. (I leave it to you to find out whether those can be followed by into, or only by to, or by no preposition at all.)

Solution 3:

It is called a metaphor, or possibly hypocatastasis depending on the way it is written.

I organize my spice rack alphabetically; I am so OCD.

Is a metaphor.

My OCD organization system puts cumin next to cinammon.

Is hypocatastasis. In both cases they compare something to OCD, and the meaning is that the "something" bears some resemblance to the other in some aspects though not all. One cannot over emphasize the similarity. It is unlikely that your anal spice rack attitude requires medication as is the case with some people with real OCD.

I remember the three basic figures of speech of comparison, simile, metaphor and hypocatastasis with memories of my childhood discussions with my mother:

Fraser, your room is like a pigsty; clean it up. (Simile, compare with "like" or "as".)

Fraser, your room is a pigsty; clean it up. (Metaphor, compare with "to be".)

Fraser, clean up your pigsty! (Hypocatastasis, compare by replacement.)

With no disrespect to our porcine friends.