(Britain, Ireland, colloquial) In an underhand way, sneakily; unofficially, particularly with regard to payment or tax avoidance.

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).

Illicit, obtained or sold furtively.