c. 1300, "a bed," also "bed-like vehicle carried on men's shoulders" (early 14c.), from Anglo-French litere "portable bed," Old French litiere "litter, stretcher, bier; straw, bedding" (12c.), from Medieval Latin lectaria "litter," from Latin lectus "bed, lounge, sofa, dining-couch," from PIE *legh-to-, suffixed form of root *legh- "to lie down, lay."
Altered in French by influence of lit "bed." The meaning was extended early 15c. to "straw used for bedding" (this sense is early 14c. in Anglo-French) and by late 15c. to "offspring of an animal at one birth" (that is, in one bed). Litter by 19c. had come to mean both the straw bedding and the animal waste in it after use. The sense of "scattered oddments, disorderly debris" is first attested 1730 and probably is from litter (v.) "provide with bedding" (late 14c.) and sense extended from the image of strewing straw.
late 14c., "provide with bedding," from litter (n.). Meaning "bring forth, give birth to" (of animals or, contemptuously, of humans) is from late 15c. Meaning "to strew with objects" is from 1713. Transitive sense of "to scatter in a disorderly way" is from 1731. Related: Littered; littering.