under the counter

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adverb 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 UK, Ireland, colloquial (Particularly with regard to payment or tax avoidance) In an underhand way, sneakily. Unofficially.