spiffed up vs dressed to kill

spiffed up and dressed to kill both are adjectives.

Word	Noun	Adjective	Verb	Adverb
spiffed up	No	Yes	No	No
dressed to kill	No	Yes	No	No

As adjectives, spiffed up and dressed to kill are synonyms defined as:

Other synonyms of spiffed up include dolled up, dressed-up, dressed, dressed to the nines, spruced up, togged up.

spiffed up (adjective)	dressed to kill (adjective)
dressed in fancy or formal clothing	dressed in fancy or formal clothing