weakly
adj.physically fragile or not very strong. You use this to describe someone who is often sick or lacks physical power.
adj.physically frail or lacking in strength. Often describes a person's constitution or physical state rather than a temporary illness.
“He was a weakly child who stayed indoors most of the time.”
“The doctor recommended a specific diet to help the weakly patient regain some muscle mass.”
“Despite his weakly appearance and soft-spoken nature, he possessed a mental fortitude that surprised even his most aggressive business rivals.”
Typically used attributively before a noun; not to be confused with the adverb form of 'weak'.
While 'weakly' is an adjective meaning frail, using it as an adverb for 'in a weak manner' is often considered awkward or non-standard in modern English; 'weakly' as an adverb is usually reserved for logical or structural contexts.
From Old English wāclīċe (“weakly”), equivalent to weak + -ly (adjectival suffix); compare Old English wāclīċ (“weak; ignoble; mean”), and Old Norse veikligr (“weakly; sick”); both ultimately from Proto-Germanic *waikalīkaz (“weakly; weak”).
From Middle English weykly, equivalent to weak + -ly (adverbial suffix). Compare Old High German weihlīcho (“weakly”), Middle English wocliche, wokli, wacliche (both from Proto-Germanic *waikalīkō).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.