knapsack (n.)

"large oblong bag," Middle English sak, from Old English sacc (West Saxon), sec (Mercian), sæc (Old Kentish) "large cloth bag," also "sackcloth," from Proto-Germanic *sakkiz (source also of Middle Dutch sak, Old High German sac, Old Norse sekkr, but Gothic sakkus probably is directly from Greek), an early borrowing from Latin saccus (also source of Old French sac, Spanish saco, Italian sacco), from Greek sakkos "bag (made of goat hair); sieve; burlap, large burlap cloak," which is from Semitic (compare Hebrew, Phoenician saq "sack, cloth of hair, bag, mourning-dress").

The wide spread of this word for "a bag" probably is due to the incident in the Biblical story of Joseph in which a sack of corn figures (Genesis xliv). In English, the meaning "a sack or sack material used as an article of clothing" as a token of penitence or mourning is from c. 1200. The baseball slang sense of "a base" is attested from 1913.

The slang meaning "bunk, bed" is by 1825, originally nautical, hence many slang phrases, originally nautical, such as sack duty "sleep;" the verb meaning "go to bed" is recorded from 1946. Sack-race (n.) is attested from 1805.