Simply that there is some form of statute (formal law passed by a parliament) which outlaws an action. If a law saws "do not drive on the left" then driving on the left is "prohibited by law". It is, to be honest, a bit of a tautology!
Law
Not prohibited by Federal law, MAY be prohibited by state law, or limits placed on parole. Check with your local law enforcement.
Prohibited drugs are drugs that aren't allowed by law.
That funds and supporting documents are available, and goods or services have been received.
no
Goods prohibited by law or treaty from being imported or exported, Illegal traffic in contraband; smuggling.
Speeding
Illicit Trade
A society in which marriage between classes is prohibited by law or custom is based on a caste system.
prohibited by law or by official or accepted rules
sports,dancing,theatre
Those which are prohibited by legislation.