The quality or state of being flimsy; lack of substance, strength, or conviction.
From 'flimsy' plus the nominalizing suffix '-ness.' This abstract noun formation converts the adjective into a concept that can be discussed and measured.
The word 'flimsiness' became especially popular in the early 20th century when people used it to criticize everything from excuses to political arguments—it's a way of calling something out as obviously inadequate.
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