sickly vs under the weather

sickly and under the weather both are adjectives.

Word	Noun	Adjective	Verb	Adverb
sickly	No	Yes	No	No
under the weather	No	Yes	No	No

As adjectives, sickly and under the weather are synonyms defined as:

Other synonyms of sickly include ailing, indisposed, peaked, poorly, seedy, unwell.

sickly (adjective)	under the weather (adjective)
somewhat ill or prone to illness	somewhat ill or prone to illness
unhealthy looking