Weak is the opposite of strong. Muscles, arguments, defenses, and coffee can all be weak — and when they are, it's not good.
Anything weak is lacking in strength or power. If your muscles are weak, you can't lift much. If your will power is weak, you give up easily. Food and drinks with little flavor are weak, and an argument that is flimsy is weak. Whenever anything is lacking or not what it should be, it's weak. And if you come across something really lame — like a dumb TV show — you could say, "That's so weak."