1
general::
In secret, as in They paid her under the table so as to avoid taxes. This term alludes to money being passed under a table in some shady transaction, such as a bribe. [Mid-1900s] Also see UNDER THE COUNTER.
American Heritage Idioms
2
general::
Phrase(s): under the table
1. Sl. intoxicated. • Jed was under the table by midnight. • By 3:00 in the morning, everyone was under the table. 2. Fig. secret; clandestine. (This is hyphenated before a nominal.) • It was strictly an under-the-table deal. • The mayor made a few bucks under the table, too.
McGrawhill's American Idioms And Phrasal Verbs