stiffness vs clumsiness

stiffness and clumsiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
stiffness	Yes	No	No	No
clumsiness	Yes	No	No	No

As nouns, clumsiness is a hypernym of stiffness; that is, clumsiness is a word with a broader meaning than stiffness:

stiffness: the property of moving with pain or difficulty
clumsiness: the carriage of someone whose movements and posture are ungainly or inelegant

Other hypernyms of stiffness include awkwardness.

As nouns, stiffness and clumsiness are synonyms defined as:

stiffness and clumsiness: the inelegance of someone stiff and unrelaxed (as by embarrassment)

Other synonyms of stiffness include awkwardness, gracelessness.

stiffness (noun)	clumsiness (noun)
the physical property of being inflexible and hard to bend	the carriage of someone whose movements and posture are ungainly or inelegant
the property of moving with pain or difficulty	the inelegance of someone stiff and unrelaxed (as by embarrassment)
the inelegance of someone stiff and unrelaxed (as by embarrassment)	unskillfulness resulting from a lack of training
excessive sternness
firm resoluteness in purpose or opinion or action