Most back formations make new words, such as edit and scavenge which would never have come about if edition and scavenger hadn't existed already, though it looks at first sight as though the longer word must have been derived from the shorter. Credible is different because it isn't a new word, but Ms Morris's (or the Guardian's) use of it seems to have been derived from the longer and more elaborate (credibility, so it has every right to be classed as a back formation too.
Credible usually means that a story can be believed, but here it has a far more specific meaning: a credible person is someone with "street credibility", and Ms Morris's complaint was that being good at football gave a pupil more street cred than being good in class.
Credibility has itself changed a bit since the days when it meant only "the quality of being believable". The change came during the Cold War when a nuclear deterrent's credibility depended on whether the other side thought it might be used. It became a political term.
American Congressmen who wondered whether the electorate was going to swallow their policies would speak of the "credibility gap" and the expression was then taken up by the public at large, with a touch of jokiness about it. This was in the 1960s.
The first instance of "street credibility" found by Oxford dictionary editors dates from 1979 and comes from the now defunct popular music magazine Sounds. "Levine," it said, "has real street credibility (not like some wimp ... who went to public school and tells the world he's as street level as the Cockney rejects)." So the person with street cred (the abbreviation came soon after) was one of the lads and lasses.
So far so good, but I now find on looking in the Oxford English Dictionary that credible, as applied to weaponry, was in fact used earlier than credibility, which seems to put paid to my theory about its being a back formation.
However, when Ms Morris talks about pupils being "credible" I very much doubt whether she, or the journalist reporting her, has nuclear deterrence in mind. The reference is to the late 1970s, not the early 1960s, and my case rests.
Scholarly readers of the Independent on Sunday may tell me how wrong all this is. Didn't Milton use the word credible back in the 1630s to mean "acceptable" and "deserving credit", and isn't this what street cred is about, at least among those who use the term? Quite right. The OED does indeed have a quote from Milton in which he uses the word in this very sense. But marks it with the sweet little dagger that means "obsolete".
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