Wiktionary
n. (context business idiomatic English) A leisurely, expensive, midday meal associated with drinking, which is tax-deductible because business is discussed.
Wikipedia
The three-martini lunch is a term used in the United States to describe a leisurely, indulgent lunch enjoyed by businesspeople or executives. It refers to a common belief that many people in such professions have enough leisure time and wherewithal to consume more than one martini during the work day. Steaks, oysters and lobster are among dishes cited as a staple of these lunches.
As business matters may be discussed at them, three-martini lunches are considered a business expense (which includes travel, meals, etc.) and thus can qualify for a tax deduction.
Usage examples of "three-martini lunch".
But it was equally possible that she had no idea that either murder had taken place and was shopping at Saks or having a three-martini lunch in honor of Jerome's departure (albeit for a destination noticeably farther than Rio).
It was the same red ten-speed that she was to be riding three days later when a prominent local businessman, rushing back to his office after a three-martini lunch, ran a stop sign.