In response to a recent Language Log post ("It is so neat and creepy", 10/19/2006), Dan Fuchs suggests that creepy is following a path blazed by terrific. He points to the online etymological dictionary entry for terrific
1667, "frightening," from L. terrificus "causing terror or fear," from terrere "fill with fear" (see terrible) + root of facere "to make" (see factitious). Weakened sensed of "very great, severe" (e.g. terrific headache) appeared 1809; colloquial sense of "excellent" began 1888.
His comment: "So it went from 'really horrible' to 'really excellent' in a mere 200 years -- and without blogs or MySpace!"
Well, the OED entry (the earliest citation for each sense is given below) suggests that it might have been more like 260 years:
1. Causing terror, terrifying; fitted to terrify; dreadful, terrible, frightful.
1667 MILTON P.L. VII. 497 The Serpent..with brazen Eyes And hairie Main terrific.
2. a. Applied intensively to anything very severe or excessive. colloq. (Cf. awful, terrible, tremendous.)
1809 J. W. CROKER in Croker Papers 12 Oct., I am..up to my eyes in business, the extent of which is quite terrific.
b. As an enthusiastic term of commendation: superlatively good, ‘marvellous’, ‘great’. Also Comb. colloq.
1930 D. G. MACKAIL Young Livingstones xi. 271 ‘Thanks awfully,’ said Rex. ‘That'll be ripping.’ ‘Fine!’ said Derek Yardley. ‘Great! Terrific!’
Still, it's creepy (in the new sense) how words change their meaning over time, and terrific is an even creepier example than uncanny is. And then of course there's the adverbial evolution of awfully...
[And as several readers have pointed out, there's awesome. The OED's senses, with the date of the earliest citation for each one:
1. Full of awe, profoundly reverential. 
2. Inspiring awe; appalling, dreadful, weird. 
3. a. In weakened sense: overwhelming, staggering; remarkable, prodigious. colloq. (orig. and chiefly U.S.). 
b. In trivial use, as an enthusiastic term of commendation: ‘marvellous’, ‘great’; stunning, mind-boggling. slang. 
"In trivial use"? ]
[Chris Christensen points out that negative connotations come as well as go, via the quote attributed to S.M. Stirling:
Words mean what they're generally believed to mean. When Charles II saw Christopher Wren's St. Paul's Cathedral for the first time, he called it "awful, pompous, and artificial." Meaning roughly: Awesome, majestic, and ingenious.
[Update 10/23/2006 -- Dave Lebling writes:
Chris Christenson's quote on "awful, pompous and artificial" is not from S. M. Stirling, but rather an earlier science fiction writer, Poul Anderson, in his short story "A Tragedy of Errors."
My recollection is that the story revolves around a planet where the word "friend" has come to mean pirate/terrorist/bad-guy due to raids by bad guys who call themselves "friends."
]Posted by Mark Liberman at October 20, 2006 06:32 PM