The quality or state of being assailable; the degree to which something can be attacked or challenged.
From assailable + -ness suffix, creating an abstract noun form. The suffix -ness, from Old English, turns adjectives into nouns describing their qualities.
This word rarely appears in modern English, but linguists love it as an example of how English lets us stack suffixes—assail → assailable → assailableness—each layer adding abstraction.
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