Hi, and sorry for the delayed response. There's no ironclad rule agaisnt it but generally Urban Dictionary would fail the policy on reliable sources because it does not "have a reputation for fact-checking and accuracy". That doesn't make the definitions there wrong per se, just potentially unreliable. A word or phrase with genuine currency would be extremely likely to also be defined somewhere other than Urban Dictionary, so anything reliant on UD alone as a source is unlikely to be acceptable.