Acting like a plant. Used to describe behavior in the classroom or workplace where one does not participate in any way, but instead just sits there, soaking up information.
My math class is easy as long as I pay attention during lecture. All this week I have done nothing but being a plant, soaking up my teacher's rays of enlightenment.
When you try talking to a friend who seemingly ignores you for days at a time and gives no attention back, and overall acts like they don't want to be your friend anymore. Just like giving water to a dead plant, the plant will never grow again or give anything back. Hence the name, watering a dead plant.
girl 1: damn, I was talking to Emily last night and it feels like I'm watering a dead plant
girl 2: ditch him, you shouldn't be friends with someone who doesn't match your energy