Present participle of facsimile used as a verb; the act of creating an exact copy or reproduction.
From 'facsimile' (noun used as verb) + '-ing' (present participle suffix). This gerund form refers to the ongoing action of making facsimiles.
The word 'facsimileing' is technically possible in English but practically obsolete—it's a perfect example of how words can follow grammatical rules but still fall out of use when technology changes what we actually do!
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