sickly vs ailing

sickly and ailing both are adjectives.

Word	Noun	Adjective	Verb	Adverb
sickly	No	Yes	No	No
ailing	No	Yes	No	No

As adjectives, sickly and ailing are synonyms defined as:

Other synonyms of sickly include indisposed, peaked, poorly, seedy, under the weather, unwell.

sickly (adjective)	ailing (adjective)
somewhat ill or prone to illness	somewhat ill or prone to illness
unhealthy looking