shamus vs private investigator

shamus and private investigator both are nouns.

Word	Noun	Adjective	Verb	Adverb
shamus	Yes	No	No	No
private investigator	Yes	No	No	No

As nouns, shamus and private investigator are synonyms defined as:

shamus and private investigator: someone who can be employed as a detective to collect information and assist in criminal investigations

Other synonyms of shamus include operative, PI, private detective, private eye, sherlock.

shamus (noun)	private investigator (noun)
someone who can be employed as a detective to collect information and assist in criminal investigations	someone who can be employed as a detective to collect information and assist in criminal investigations