under the weather vs ailing

under the weather and ailing both are adjectives.

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

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

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

under the weather (adjective)	ailing (adjective)
somewhat ill or prone to illness	somewhat ill or prone to illness