Definition
A leisurely, expensive midday meal associated with drinking, which is tax-deductible because business is discussed.
How People Actually Use It
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
"Bob and Susan took their clients out for a three martini lunch to discuss future products releases."
Source: Urban Dictionary