Tweed is a cold weather fabric from Europe. It's got roots in hunting, then evolved into a wardrobe staple that kept dudes looking warm and classy. A holdover from the days when dressing "casual" meant not wearing a tuxedo, but still looking respectable with a tucked-in shirt and probably a tie.
Over time tweed has been reinterpreted into many forms, to slim-fitting sportcoats and trousers and outerwear like jackets, and even sneakers. Learn yourself something new today and check out The Complex Guide to Tweed.