You're lugging the engine when you don't accelerate immediately while pressing the gas pedal. For instance, if your RPM is at 2,500, traveling 40 mph, and you're in 5th gear and trying to accelerate, you're lugging the engine. Also, the engine is lugging if you're in 2nd gear and attempt to increase speed when your RPM is at 1800 traveling 10 mph. Take note that different vehicle models have different RPMs.