In an experimental manner; through or by means of experiments. (Note: This word is extremely rare and may be non-standard.)
Theoretically from experiment + -ly (adverbial suffix), though this form is archaic or non-standard. The suffix -ly normally attaches to adjectives, and 'expermental' would be the expected adjective form from which this would derive. The standard form 'experimentally' is far more common.
This word is so rare that even comprehensive dictionaries barely list it—it's a reminder that not every possible word combination in English actually gets used by real people, and languages naturally prefer shorter, more elegant forms!
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