1769, "broad street or promenade planted with rows of trees," from French boulevard, originally "top surface of a military rampart" (15c.), from a garbled attempt to adopt Middle Dutch bolwerc "wall of a fortification" (see bulwark) into French, which at that time lacked a -w- in its alphabet.
The notion is of a promenade laid out atop demolished city walls, a way which would be much wider than urban streets. Originally in English with conscious echoes of Paris; in U.S., since 1929, used of multi-lane limited-access urban highways. Early French attempts to digest the Dutch word also include boloart, boulever, boloirque, bollvercq.