back pack vs packsack

back pack and packsack both are nouns.

Word	Noun	Adjective	Verb	Adverb
back pack	Yes	No	No	No
packsack	Yes	No	No	No

As nouns, back pack and packsack are synonyms defined as:

back pack and packsack: a bag carried by a strap on your back or shoulder

Other synonyms of back pack include backpack, haversack, knapsack, rucksack.

back pack (noun)	packsack (noun)
a bag carried by a strap on your back or shoulder	a bag carried by a strap on your back or shoulder