1
general::
Phrase(s): under the counter
Fig. [bought or sold] in secret or illegally. (Compare this to over the counter.) • The drugstore owner was arrested for selling liquor under the counter. • The clerk sold dirty books under the counter.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Secretly, surreptitiously, as in I'm sure they're selling liquor to minors under the counter. This expression most often alludes to an illegal transaction, the counter being the flat-surfaced furnishing or table over which legal business is conducted. It was first recorded in 1926. Also see UNDER THE TABLE.
American Heritage Idioms