The quality of being firmly resolved or determined; a state of being sure about a decision or course of action.
From 'decided' (the adjective meaning firm or determined) plus the suffix '-ness' to form an abstract noun. It's a straightforward formation in English.
Someone with 'decidedness' is rock-solid in their choices—they don't flip-flop—which is actually a rare quality in a world where people keep changing their minds!
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