An evening slack is a term derived from the eponymous label of the creature commonly known as the evening slacker.
Originally, in the early 19th century, primarily in London, there were men of reduced means who would attach themselves to elegant women entering fancy restaurants. Their motives can only be imagined.
In defense, the women began sporting man-like clothing, but looser, more feminine versions, in order to deflect the attention of these less-than-desirable clingers on.
Over time, that particular dress became known as the slacker, or for brevity's sake, the slack.
The tradition has really died off, except in pockets of conservative Protestant culture. Rumor has it that the term is still used in a few resorts in the northeastern United States.
Please note that during dinner, jackets are required for gentlemen age twelve and over, and for ladies, dresses, skirts or evening slacks.