Greek "asthenei" means to be lacking in strength or power, translated well as "infirmus" into Latin but perhaps without the same connotation. It implies incapacity, something debilitating, rather than simple illness.
Ordinary terms that mean one thing in the colloquial language quickly took on theological meaning in the Church. "presbyter" colloquially meant "old man," or "elder," but very early the Church applied that to young ordained priests. "episcopos" means "overseer" but it had a specialized meaning. "charis" meant "favor", thus the crappy "o higly favored one" modern false translations of "full of grace" for Our Lady, but "charis" clearly had a technical theological meaning very early. It's like our use of the word "grace". In colloquial language it has to do with elegance or beauty, but when we say "grace of God", we're referring to a term with a very specific technical meaning. If someone 1,000 year from now after English was no longer spoken discovered a text referring to the grace of God, and started translating it as "the elegance of God," they'd be absurdly wrong. That's exactly what's being done by the faithless Modernists with the texts of Sacred Scripture.
So we refer only to Tradition to help us understand what terms like "old man" (presbyter), "overseer" (episcopos), "grace" (charis), and here "infirm" (asthenos) mean in the theological language of the Church.
Don't be fooled by the Modernist attempt to colloquialize the language of the early Church. I had planned to write my Doctoral Dissertation on this very subject after I had completed Ph.D coursed at Catholic University, but my life took a different term and I never completed the Doctorate.