In a way that can be converted or changed from one form to another.
Convertible + -ly (adverbial suffix). An extremely rare adverbial formation from the adjective convertible, following standard English word-formation patterns.
This word is so uncommon that most native speakers would never use it—they'd say 'in a convertible manner' instead. It's a perfect example of how English allows word formation but doesn't always use every possible combination.
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