The quality of being combinable or the state of being able to be combined.
From 'combinable' plus '-ness' (from Old English, meaning 'state of being'). This creates an abstract noun describing the quality itself, though it's quite rare in modern usage.
Adding '-ness' to an '-able' adjective feels redundant to modern ears — it's like saying 'ability-ness' — which is probably why you almost never see this word used in practice anymore.
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