The story
From jerk (“to move with a sudden movement”) + water. Refers to the need to supply the boilers of steam trains with water. In rural areas and small towns with no water tower, where the train did not stop, this was done by scooping ("jerking") water from a track pan. First appears c. 1852, in the Miami County Sentinel (Peru, Indiana).
Etymology from Wiktionary, CC BY-SA 4.0.