slacking vs goofing off

slacking and goofing off both are nouns.

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

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

Other synonyms of slacking include goldbricking, shirking, soldiering.

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