stern vs unforgiving

stern is a noun but unforgiving is not a noun.

Word	Noun	Adjective	Verb	Adverb
stern	Yes	Yes	No	No
unforgiving	No	Yes	No	No

As adjectives, stern and unforgiving are synonyms defined as:

stern and unforgiving: not to be placated or appeased or moved by entreaty

Other synonyms of stern include grim, inexorable, relentless, unappeasable, unrelenting.

stern (noun)	unforgiving (noun)
the rear part of a ship
the fleshy part of the human body that you sit on

stern (adjective)	unforgiving (adjective)
severe and unremitting in making demands	unwilling or unable to forgive or show mercy
not to be placated or appeased or moved by entreaty	not to be placated or appeased or moved by entreaty
of a stern or strict bearing or demeanor; forbidding in aspect
severely simple