first baseman vs first sacker

first baseman and first sacker both are nouns.

Word	Noun	Adjective	Verb	Adverb
first baseman	Yes	No	No	No
first sacker	Yes	No	No	No

As nouns, first baseman and first sacker are synonyms defined as:

first baseman and first sacker: (baseball) the person who plays first base

first baseman (noun)	first sacker (noun)
(baseball) the person who plays first base	(baseball) the person who plays first base