In an illicit way, i.e. not permitted by the law or regulations.
In a secret or covert manner.
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.