Archaic or variant form possibly intended to mean heaviness; an obsolete noun formation no longer in standard use.
Possibly from 'heavy' + '-ity' (suffix creating abstract nouns from adjectives). The '-ity' suffix comes from Latin '-itas' and Greek '-ites,' used extensively in English for creating nouns from adjectives (like 'gravity' from 'grave'). If this word existed historically, it would have competed with 'heaviness.'
English has two main ways to nominalize adjectives: '-ness' and '-ity,' but they're not interchangeable—we say 'heaviness' not 'heavity,' while we say 'gravity' not 'heaviness'! It's a historical accident which one won out for each adjective.
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