off his guard vs napping

off his guard and napping both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off his guard	No	Yes	No	No
napping	No	Yes	No	No

As adjectives, off his guard and napping are synonyms defined as:

Other synonyms of off his guard include off-guard, off guard, off her guard, off one's guard, off your guard.

off his guard (adjective)	napping (adjective)
not prepared or vigilant	not prepared or vigilant