at work vs busy

at work and busy both are adjectives.

Word	Noun	Adjective	Verb	Adverb
at work	No	Yes	No	No
busy	No	Yes	Yes	No

at work (adjective)	busy (adjective)
on the job	actively or fully engaged or occupied
	overcrowded or cluttered with detail
	intrusive in a meddling or offensive manner
	(of facilities such as telephones or lavatories) unavailable for use by anyone else or indicating unavailability; (`engaged' is a British term for a busy telephone line)
	crowded with or characterized by much activity

at work (verb)	busy (verb)
	keep busy with