fastball vs hummer

fastball and hummer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastball	Yes	No	No	No
hummer	Yes	No	No	No

As nouns, fastball and hummer are synonyms defined as:

Other synonyms of fastball include bullet, heater, smoke.

fastball (noun)	hummer (noun)
(baseball) a pitch thrown with maximum velocity	(baseball) a pitch thrown with maximum velocity
	a singer who produces a tune without opening the lips or forming words