A story, usually only slightly exaggerated over time, told by sweats (mostly of the Royal Marines variety) to stiffen the crow's upper lip. Whatever happened in the 'dit' is always much worse than the situation at this very moment; if I was trapped in a cage with a crocodile and a bootneck, I'd be willing to bet that his last words would be 'pah, when we were in the Falklands, we got trapped in a cage with TWO crocodiles!'.
There's a great line in some war film that my granddad used to watch all the time:
'Corporal!' 'Yes sir?' 'In future, whenever I say 'chin up, we've been in worse before' - this is what I'm talking about!'
Source of the British squaddie's nickname 'Wennie' because they all start with 'when I [was in]'