Why do dust bins have the phrase "Do not Litter"? I checked the dictionary meaning - litter means garbage or waste. Aren't dustbins meant for that ?
I'm guessing that, when you checked the dictionary, you focused on the word's meaning as a noun. If the signmaker's intent was to prevent you from putting litter in the dustbin, then the sign would read "No Litter" or "Not for Litter" (or maybe "Recyclables Only").
Instead, the word is being used as a verb. As a verb, litter means "to scatter around a certain place," or "to strew with garbage" (see Definitions 8 and 9 of litter).
So, the sign is exhorting citizens to use the trashcan, as opposed to throwing their rubbish on the ground.
Litter is trash laying around on the ground.
The message on the trash bin do not litter is telling you not to trow your trash just anywhere, put it in the garbage can.