Knicker-gripping stuff

Have you heard about the British couple who woke up to find they had been locked in their hotel?

Ed Potterton and Helen Day woke up late on Sunday morning to discover that their hotel, the Galfi in Ibiza had been closed down for the season - and it was only because they triggered a burglar alarm that someone came to get them. What I find really strange is not being so much being abandoned in the hotel - I've slept through more dramatic things to be honest - but Helen Day's response: "I was frightened at first. When I thought it was the burglar alarm I was frightened. My knees were jelly. Being locked inside the hotel after it had closed down for the season was absolutely knicker-gripping." Knicker-gripping?

What does it mean? Did she have particularly large knickers and use a couple of pairs to wave 'SOS' in semaphore? Or is it merely 'buttock clenching' for those of a genteel disposition?