This question was originally posted here: What is the term for a word that has come to mean the opposite of its orignal meaning?
I don't think it should have been marked as a duplicate: the "duplicate" question was asking about auto-antonyms/contronyms; however, this question refers to words that no longer hold their original meaning (I'm almost sure, necessarily via misuse). Whereas contronyms can mean two, opposite things, the type of words we're asking about no longer hold two meanings; they just mean one thing opposite to what they originally meant.
[This question might be better for meta. In which case, I'd appreciate if somebody with the requisite points posts it there].
I came across this question while wondering about the verb "Nonplus," or specifically about the passive participle, "Nonplussed." I've (mis)used the word, intending to mean "not at all affected or surprised," and have been understood, although it means the exact opposite.
So I'm re-opening this question!
Update to clarify some good points brought up in the comments to the initial post:
I'm referring to words whose actual meanings over time have changed to their initial meaning's opposites, not those whose connotations have switched from positive to negative or vice versa (as with "condescension," as Phil mentioned, or "innovation").
Another comment asked if there was an example of a word that lost its old meaning entirely. I think I found one: "hussy," according to this article, used to mean the mistress of a household, or housewife, which, to my mind, is an antonym for what it now means. A better example is the phrase "hoity-toity." See this link for more on Michael Quinion's blog. I would post more links, specifically, to issues 802, 803, 851, and 852 of his Newsletter for mentions of "hoi polloi," whose misuse to mean the upper classes is becoming more prevalent, but I don't have enough points to post more than two links...