I am mowing the lawn or the grass, if you mean using a machine [lawn mower] The manual instrument used a long time ago (that you can see in the hands of the grim reaper) is a scythe. There's also something called a "weed whacker", a type of machine (google it for pictures). But you'd still say I was using my weed whacker to mow the lawn, NOT "I was whacking my weeds" :)