slap-up vs old

slap-up is not a noun while old is a noun.

Word	Noun	Adjective	Verb	Adverb
slap-up	No	Yes	No	No
old	Yes	Yes	No	No

As adjectives, slap-up and old are synonyms defined as:

Other synonyms of slap-up include bang-up, bully, corking, cracking, dandy, great, groovy, keen, neat, nifty, not bad, peachy, smashing, swell.

slap-up (noun)	old (noun)
	past times

slap-up (adjective)	old (adjective)
excellent	of long duration; not new
	(used especially of persons) having lived for a relatively long time or attained a specific age
	skilled through long experience
	(used for emphasis) very familiar
	excellent
	just preceding something else in time or order
	belonging to some prior time