An excuse for police to rowst citizens they wish to hassle or move along. The original sense of mope was to wander aimlessly (the sense of to be bored or depressed came later), and some jurisdictions actually put misdemeanor laws on the books that criminalize what is in effect "loitering while walking." (Yeah, normally a condition of loitering is not actually moving, so IDK.) Despite there being actual an legal definition, this is marked by many dictionaries as slang, or at least informal, and specifically US slang. As for mope itself, it's Tudor slang from the 16th century of unknown origin.
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