To my mind, the literal definition is the previous one "to plod heavily" (as in through snow) and it has, over time, idiomatically come to mean any kind of hard

To my mind, the literal definition is the previous one "to plod heavily" (as in through snow) and it has, over time, idiomatically come to mean any kind of hard, slow work. The metaphor has come to roost in the dictionary, so to say. (-: