You can describe weak, thin, and fragile things as flimsy. Onion smells are strong, onion skins are flimsy. Blaming onions for making you cry during a sad movie, that's a flimsy excuse, when there are no onions in the theater.
A flimsy object and a flimsy objection or excuse are both without weight and are easy to knock down. You can see right through a flimsy curtain, and you can see right through a flimsy lie. If a tent or house of cards is flimsy, it will fall down with a slight breeze, and if your story isn't convincing anyone, it's probably weak and flimsy enough to flatten, too.