Anything that happens under-the-table is taking place secretly, like under-the-table payments that are illegal.

The things on a table are easily seen, but it's not as easy to see what's happening under the table. That can help you remember that under-the-table activities are secret, covert, confidential, and sometimes illegal. If two businesses have an under-the-table agreement, no one else knows about it. If a company pays someone under-the-table, the money is not documented and therefore won't be taxed. Just about everything spies do happens under-the-table.

Definitions of under-the-table
  1. adjective
    designed and carried out secretly or confidentially
    “has their under-the-table backing”
    synonyms: behind-the-scenes, sub-rosa
    secret or hidden; not openly practiced or engaged in or shown or avowed
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