The quality or state of being flabby; looseness and lack of firmness, especially of flesh.
From flabby plus -ness suffix, creating an abstract noun; flabby itself comes from flab (loose flesh), a term that emerged in English in the 19th century.
The word 'flab' was originally slang for loose, unwanted flesh, but it became so useful and catchy that it evolved into respectable dictionary words like flabbiness, showing how informal language can become formal.
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