second-stringer vs jock

second-stringer and jock both are nouns.

Word	Noun	Adjective	Verb	Adverb
second-stringer	Yes	No	No	No
jock	Yes	No	No	No

As nouns, jock is a hypernym of second-stringer; that is, jock is a word with a broader meaning than second-stringer:

second-stringer: an athlete who plays only when a starter on the team is replaced
jock: a person trained to compete in sports

Other hypernyms of second-stringer include athlete.

second-stringer (noun)	jock (noun)
an athlete who plays only when a starter on the team is replaced	a support for the genitals worn by men engaging in strenuous exercise
	a person trained to compete in sports