late 14c., remaindre, in law, a right of ownership designed to devolve upon a second party, from Anglo-French remeinder, Old French remaindre, noun use of infinitive, a variant of Old French remanoir "to stay, dwell, remain; be left; hold out," from Latin remanere "to remain, to stay behind; be left behind; endure, abide, last" (source also of Old Spanish remaner, Italian rimanere).
The general meaning "that which remains, anything left over after separation, removal, etc." is by 1550s. In mathematics from 1570s. Specifically in publication, "what remains of an edition the sale of which has practically ceased and is sold at a reduced price" (1757).
early 15c., in law, of rights of ownership, "to devolve upon a second party," from remainder (n.). The meaning "dispose of (the remaining unsold editions of a book) at a reduced price" is by 1902, from the noun in the publication sense. Related: Remaindered.