rhyming slang - bottle and glass - a*se. Lose your a*se suggests you're so scared you lose control of your bowels. Bottled out is a more recent version of the same idea.
Thanks for your replies, Schutz,TCL and Quizmonster. At last,an explantion! I've been asking around but nobody knew. Thanks for the link Quiz,I laughed at the bit about Gordon Brown. Oh I know what a lexipographer is,I just googled it..lol Thanks again. china