My posting on formations in -tard elicited some Facebook comments that were facetious (reminders of leotard and custard) and some that were dismayed at the offensiveness of these formations and the noun retard from which they derive. All this stuff is indeed offensive and is so labeled by lexicographic sources.
And behind this lies a nasty jungle of technical terms, euphemisms, semantic shifts, lexical replacements, specific slurs, and generalized insults — such a tangle that there’s no easy way to even talk about the semantic domain in question, which has to do with what once was called mental defectiveness (itself a technical term covering various vernacular terms) and then mental retardation or simply retardation (with the image of slowness or being held back). The corresponding adjectival formations are mentally retarded or (later, in another euphemistic move) mentally challenged.
That brings us to the adjective retarded, used as a technical term for being mentally retarded, then (through a shift that occurs again and again) as a vernacular slur against such people, and then (through another shift that occurs again and again) as a generalized slur against people or actions that are viewed as defective or unsatisfactory in any way whatsoever. Retarded begets the back-formed noun rétard (note the accent pattern), again both a specific and a generalized slur, and the clipped variant tard, with the same uses.
Then new technical terms arise — notably special education or special ed for programs designed to aid the people in question — in the conflict between those who thoughtfully avoid giving offense and those who deliberately choose to give offense. Special ed gets abbreviated as SPED, and then (predicably) the noun sped develops a life as a specific insult and also as a generalized insult, directed at people. (Separately, the adjective mental picks up uses not only as ‘insane, crazy’, but also as ‘silly’ and as ‘retarded’ as a generalized slur.)
Then there are expressions designed to take in mental retardation and often physical disability as well (in a higher-level category that not everyone is comfortable with): special needs, and accompanying it, shortbus or short bus for the truncated buses used to convey special needs students to school in some parts of the U.S., and then (as a slur) for someone who rides on them. (In an appalling side development, there’s window licker, for those riders, viewed as pressed against the windows of their shortbuses to take in the scene outside.)
This is from a U.S. viewpoint. There’s more to be said if you take in other parts of the English-speaking world; I’m not attempting to survey all the extant usages in this domain, just sampling some characteristic developments.
[There are older usages as well: feeble-minded (truncated to the slur feeb), dumb, fool, stupid, and the sequence moron, imbecile, and idiot (as technical terms, denoting levels of retardation from moderate to severe), the latter two taken from ordinary language, all three then returned to ordinary language, where they've had a rich life as slurs.]
This is one of those domains where running fast can do no better than keep you in place: every new expression (devised by one means or another) will almost surely be contaminated by association with the denotations in the domain, and so will be converted (in one way or another) to use as a specific slur and then as a generalized slur.