Operating according to some law or fundamental principle.
Conforming to, or recognised by the laws of society.
A character having a lawful alignment.
Illicitly, against regulations, of goods kept under the serving counter in a shop to be unobtrusively passed to a customer who knows they are available for surreptitious sale (e.g. pornographic magazines in a newsagent).
In an underhand way, sneakily; unofficially, particularly with regard to payment or tax avoidance.