In a manner relating to or characteristic of an aneurism; in the way that an aneurism affects or manifests in the body.
From aneurismal (the adjective describing aneurism-related conditions) plus the adverb suffix -ly, creating the adverbial form for describing actions or states related to aneurisms.
This is an extremely rare adverb that you'd almost never encounter in modern English—it's the kind of hyper-technical medical term from the 1800s that shows how doctors tried to make precise language, but it was probably more confusing than helpful.
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