'Caught short' is an English idiom meaning to have run out of something or to be missing something.
"Better stock up, we don't want to be caught short!"
However, in England this can also be used to mean realizing that you need to use the restroom when none is near, or in some cases, to refer to having soiled oneself. However, the most common usage appears to be as an excuse for why one is relieving oneself in the alleyway.