Adverb

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).
   • 1969 The Seven Minutes: A Novel^{http://books.google.co.uk/books?id=RUo_AAAAIAAJ&q=%22under+the+counter%22&dq=%22under+the+counter%22} - Page 157 by Irving Wallace

     Oh, yeah, I remember, you mean about me not trying to sell from under the counter?

   • 2004, James Carlos Blake, Under the Skin,^{http://books.google.com/books?id=fDeZe9KuC4gC} HarperCollins, ISBN 0060542438, page 55,

     When Prohibition became the law, they produced the stuff in greater quantity and sold it under the counter to anybody who wanted it.

Adjective

1. (Particularly with regard to payment or tax avoidance) In an underhand way, sneakily. Unofficially.

   2003 - This morning he said he could not remember exactly where or how the invoice was written. It was used to cover up an under the counter payment from the sale of his land in Lucan. (RTE News, 19 Sept 2003) http://www.rte.ie/news/2003/0919/planning.html