The Baltimore Chop is so named because its origins were in the late 1800s Baltimore Orioles. This blog post by Roar34 discusses it in some detail, along with the other antics the Orioles got up to back then. Unfortunately the article that blog post references doesn't seem to exist anymore (an article by Dr. David Haus of Bowling Green). Baseball Reference's definition also agrees with that origin (and the general concepts of the definition below).
A Baltimore Chop is a ball hit into play that is hit nearly directly down, typically landing right in front of the plate, and then bouncing fairly high up, preventing the catcher from instantly fielding it (and possibly going, slowly, towards the other infielders). According to the post above, the original Chop was a result of the Orioles placing cement in that part of the infield (under the dirt) to cause higher bounces.
This was primarily effective in the 'dead ball' era, and post-1919 there are better ways to get singles.