under the counter

Short definition from Wiktionary

Adverb

1. 1
   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).

Adjective

1. 1
   In an underhand way, sneakily; unofficially, particularly with regard to payment or tax avoidance.
2. 2
   Illicit, obtained or sold furtively.