mid-14c., "table where a money lender does business," from O.Fr. contouer "counting room, table of a bank," from M.L. computatorium "place of accounts," from L. computare (see compute). Generalized 19c. from banks to shops, then extended to display cases for goods. Countertop is attested from 1878. Phrase under the counter is from 1926.
"go against," early 14c., from O.Fr. countre "facing opposite" (see counter-).
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with under the counter
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