Jack Knife

I normally spend a bit of time at the end of the day just doing a few clues in the Saturday "Times Jumbo" cryptic crossword, and can generally persuade it to last at least half the week.  Last Saturday's was complete by Tuesday evening . . . almost.  One clue remained - "Predator with strong teeth and jack knife (4)".  My immediate thought was 'pike' (as I had already the i and e), but I couldn't relate that to "jack knife", and my usually very reliable dictionary (which I dislike having to use) was in this case no help to me at all.  So I left the crossword on one side until today.

There are three basic meanings to jack knife - first of all, the knife itself, with folding blades, secondly, what an articulated lorry may do, unwanted, when things get out of control, and thirdly, a style of competition dive.  Taking the crossword up again this afternoon in an idle moment, I toyed with the word 'dive' for a while, but couldn't relate that to the first part of the clue, which was, it seemed to me, the "twin definition" form of cryptic clue, in which the clue as a whole is cryptic, but each component is in fact non-cryptic.

Then I got sidetracked into the lyrics of 'Mack the Knife', in which predators, teeth and jack knives feature strongly . . .

Oh, the shark, dear, has such teeth, dear,

and he keeps them pearly white.

Just a jack knife has old MacHeath, dear,

and he keeps it out of sight.

Sadly, that didn’t prove fruitful! 

Having, as I do, a sincere and strongly held fear of heights, which certainly includes high-diving boards, I did not know until this afternoon that 'pike' is a diving term, and therefore my correct answer.  A jack knife is . . . "the front-dive pike, in which the body folds and unfolds", as Wikipaedia assures me (and who am I to disagree).  Well, it's good to learn something new each day, and I like the way in which a good puzzle can help do this.