The term refers to the culture of leaving work at mid-day, to go to a nearby pub / strip club and have a dozen alcoholic drinks or shots, until the evening, before leaving for home or going to a prostitute for sex. It is also considered common-practice to put the bill on the Corporate Credit Cards.
The term originates in view of the exorbitant, careless, even behaviour usually characterised by bankers and traders, at the expense of their clients. It is closely related to the word Banker's Dozen, with strip clubs and sex being the only difference.
It's a Friday, let's do a Trader's Dozen with the team.