screen door on a submarine

1.
North AmericaUnited Statesinformalhumorouscolloquial
- A vivid simile for something totally ineffective or inappropriate for its situation; utterly useless at fulfilling its intended purpose.
- An emphatic metaphor indicating that a device, plan, or arrangement fails spectacularly because it is fundamentally unsuited to its context.
2.
North AmericaUnited Statesinformalderogatorycolloquial
- A jocular insult likening a person to being extremely stupid or lacking common sense.

Examples: