To my mind this makes more sense: " Weak as dishwater" because dishwater is when you're cleaning your plates, when you're washing up, it's that water that's left, slightly grey, dirty-looking kind of water, that's what you get left.

在我看来，这更有意义：“像洗碗水一样弱” ，因为洗碗水是当你清洁盘子时， 当你洗碗时，剩下的水，略带灰色，看起来很脏的水， 这就是你的意思离开。