goofing off vs slacking

goofing off and slacking both are nouns.

Word	Noun	Adjective	Verb	Adverb
goofing off	Yes	No	No	No
slacking	Yes	No	No	No

As nouns, goofing off and slacking are synonyms defined as:

Other synonyms of goofing off include goldbricking, shirking, soldiering.

goofing off (noun)	slacking (noun)
the evasion of work or duty	the evasion of work or duty