This word was marked as rare in dictionaries a century ago and has become even more so since, though it retains a niche in elevated or pretentious prose in the sense of something lean, shrivelled, or excessively thin. It’s from Latin macilentus, lean.
In 1851 a writer evoked with it a gaunt victim of tuberculosis: “of whom I could recollect nothing but a macilent figure, stretched upon a sofa and scarcely breathing”.
It can also have a figurative sense that refers to poor-quality or inferior writing. A reviewer of Britney Spears’s album In the Zone in 2003 described it as “Britney’s most personal statement. Because it’s as lost and macilent and alluring and eager to please and disturbingly empty-eyed as she is.”
This is a slightly older example in a similarly figurative vein:
Had no schoolmaster in moments of heroic enthusiasm attempted to pound a few rules of rhetoric through my incrassate skull? Had I never heard of taste? Was the word “style” outside my macilent vocabulary?
Greener Than You Think, by Ward Moore, 1947.