A heavy shirt, which is often worn in place of a jacket during the Fall or the Spring seasons. The name originates from "jacket" and "shirt". A jac shirt may have buttons or a zippered front. A jac shirt usually has two breast pockets and may have front hand pockets as well. Jac shirts are usually worn indoors and outdoors alike.
I found this in an article about a summit meeting.
It might be remarked in passing that there was one little detail about the published photographs of the heads which would have struck Guyanese in particular, and that was their dress. At another famous meeting in Cancún, Mexico in 1981, when 23 leaders from the developed and developing nations came together in the company of Ronald Reagan and Margaret Thatcher (who was attired for her formal photo in a regal purple dress), the leaders were in general clad more or less informally for the era. However, there were two – Julius Nyerere of Tanzania and Forbes Burnham of Guyana – sporting short-sleeved shirt-jac suits.
This time around, against an almost identical sea-and-sand backdrop, nearly all the Latin leaders were decked out in what we would recognize as long-sleeved shirt-jacs. So here it is, the shirt-jac has fallen into disrepute in Guyana, and is being adopted for certain occasions (almost certainly under a different name and via a different route) by the Latin American heads.