Tonkatsu is a mixture of the Japanese word for pork (ton) and an abbreviation of the word cultlet (katsu-retsu).
Cutlets themselves were popular in the mid-Meiji period (1880's). They had to be fried a few times, though, and that complicated their preparation. The basic cutlet is a thin slice of meat dipped in flour, beaten egg and bread crumbs then sauted in a frying pan. The Japanese added thinly-sliced cabbage (which helps the body digest fat and protein) and cover both cabbage and pork with a mix of spiced worcestor sauce and ketchsup.
Fat-free pork is now generally used (and it has B-1). It was the most popular food at western-style restaurants from the late 20's through the late 80's.