The act of producing leaves or leafing out; the present participle of 'foliage' when used as a verb, though rare.
From 'foliage' used as a verb (to leaf out or produce foliage) plus '-ing,' the present participle suffix, creating a gerund form.
This is an extremely rare verb form that you'd almost never encounter—linguistically it's what happens when nouns dream of becoming verbs, and sometimes they just don't fit quite right!
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