From Longman Dictionary of Contemporary Englishsponge bagˈsponge bag noun [countable] British English 
DHa small bag for carrying the things that you need to wash with SYN toilet bag
DHa small bag for carrying the things that you need to wash with SYN toilet bagExamples from the Corpus
sponge bag• It's like being zipped into a sponge bag - yuck.• She took her sponge bag into the bathroom, expecting something primitive, and paused, surprised yet again.• He had a quick look in Dale's sponge bag.• When that final whistle went I just wanted to throw up in the sponge bag. 