Alternate Definition:
A leisure lunch taken with other proffessionals for the purposes of buisness. Often with clients or coworkers. 3 martini's = 3 hours.

Modern equivlant is two martini lunch which means to have a short three martini lunch

The three martini lunch, otherwise known as the alcoholics version of a two martini lunch. Generally used to refer to people who have too much time and nothing to do with it (see example).