poorly其实也还是一个形容词，sickly主要的用法是形容词

bac

Aunt, my boy's poorly, and I haven't been able to cure him.
这里的poorly，其实是个形容词。但是这个形容词，往往不能放在名词前面，也就是，只用作表语，表示身体不好。

poorly

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poor∙ly¹ / ˈpurlɪ ; ˈpɔːli /
◙adv
1. badly
• 糟糕地，不好地，拙劣地，蹩脚地:
»Jana's doing poorly in school.
  贾娜功课不好。
»poorly educated workers
  文化程度低的工人
»The article was poorly written.
这篇文章写得很差。

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poorly²
◙adj [BrE informal]
1. ill
• 身体不舒服的，健康不佳的:
»Matt's wife's been very poorly.
  马特的妻子身体一直不好。


poorly
poor·ly / ˈpɔːli; ˈpuəli; NAmE ˈpurli; ˈpɔːrli /

adverb   in a way that is not good enough
   糟糕地；差强人意；不足
SYN   badly :
   a poorly attended meeting (= at which there are not many people)
   寥寥几个人参加的会议
   poorly designed
   设计不周的
   The job is relatively poorly paid.
   相对而言,这工作报酬很低。
   Our candidate  fared poorly in the election (= did not get many votes).
   我们的候选人在选举中得票不多。

adjective [not usually before noun]
   (BrE, informal)  ill / sick
   有病；不适；不舒服:
   She felt poorly.
   她感到身体不适。




还有个类似的例子，sick是形容词，其实sickly也是个形容词。
sickly

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sick∙ly / ˈsɪklɪ ; ˈsɪkli /
◙ adj
1. a sickly person or animal is weak, unhealthy, and often ill
• 〔人〕 虚弱的，不健康的，常生病的:
»a sickly child
一个体弱多病的孩子
»She looked pale and sickly.
她看上去脸色苍白，身体虚弱。
2. [especially BrE] a sickly smell, taste etc is unpleasant and makes you feel sick
• 〔气味、味道等〕令人恶心的:
»A sickly smell clung to his clothes and hair.
他的衣服和头发上有一股令人恶心的气味。
3. a sickly colour or light is unpleasantly pale or weak
• 〔颜色或光线〕苍白的，微弱的:
»The walls were painted a sickly green.
  墙上刷了一种惨兮兮的绿色。
»a pale, sickly moon
苍白昏暗的月亮

♦ sickly
adv:
»a sickly sweet perfume
  甜腻的香水