Meaning

noun an accumulation of jobs not done or materials not processed that are yet to be dealt with (especially unfilled customer orders for products or services)

noun the large log at the back of a hearth fire

noun something kept back or saved for future use or a special purpose

originFrom back + log. 1680s; originally a large log at the back of a fire. Figurative sense from 1880s, meaning "something stored up for later use". Possibly influenced by logbook as well.

Etymology from Wiktionary, CC BY-SA 3.0