1. the quality of being thin, light, or insubstantial in material or construction
2. the quality of being weak, unconvincing, or lacking substance in argument or reasoning

Examples

• The flimsiness of the cardboard box made it unsuitable for shipping heavy items.
• Critics pointed out the flimsiness of the argument presented in the report.
• The flimsiness of the fabric meant it would tear easily during washing.
• The flimsiness of his excuse didn't convince anyone.
• The tent's flimsiness became apparent when the wind picked up.
• The flimsiness of the evidence made it difficult to prove the case in court.