Real Dictionary

under the counter
secretly. The usual American expression is under the table.
phrase: If someone buys or sells goods under the counter, they buy or sell them secretly and illegally.

under the counter in American English
in a clandestine manner, esp. illegally
in a surreptitious manner
adjective: sold clandestinely

under the counter in British English
(of the sale of goods, esp goods in short supply) clandestine, surreptitious, or illegal; not in an open manner
adjective

under-the-counter in American English
done, sold, given, etc. secretly in an unlawful or unethical way