From Longman Dictionary of Contemporary Englishunder the counterunder the counterSECRETif you buy something under the counter, you buy it secretly and usually illegally It’s risky, but you can get alcohol under the counter. → counter
Examples from the Corpus
under the counter• There's pots to wash and a broom under the counter.• Yet we all know that it goes on - under the counter, as it were.• That box of spark plugs under the counter is a cache.• This time you bring back a whole box of plugs to put under the counter.• Then he reached under the counter for his slim green ledgers.• And they sell under the counter, you know.• Without waiting to lift the flap she slipped under the counter.under-the-counterˌunder-the-ˈcounter adjective informal SCCSECRETunder-the-counter goods are bought or sold secretly, especially because they are illegalExamples from the Corpus
under-the-counter• Nothing under-the-counter about the six Salomes.• Cynics would suggest that an under-the-counter version of this system still applies - but they would be wrong. 