deathly vs extremely

deathly is an adjective but extremely is not an adjective.

Word	Noun	Adjective	Verb	Adverb
deathly	No	Yes	No	Yes
extremely	No	No	No	Yes

As adverbs, deathly and extremely are synonyms defined as:

Other synonyms of deathly include exceedingly, super.

deathly (adjective)	extremely (adjective)
having the physical appearance of death
causing or capable of causing death

deathly (adverb)	extremely (adverb)
to a degree resembling death	to an extreme degree
to an extreme degree	to a high degree or extent; favorably or with much respect