The origin of the term backlog meaning:
- a reserve or accumulation, as of stock, work, or business: a backlog of business orders.
probably refers to either the shipping term:
the meaning "arrears of unfulfilled orders" (1932) might be from, or suggested by, log (n.2), that is:
"record of observations, readings, etc.," 1842, sailor's shortening of log-book "daily record of a ship's speed, progress, etc." (1670s). The book so called because a wooden float at the end of a line was cast out to measure a ship's speed. (Etymonline)
or to a large piece of wood in the fireplace:
- This word is used to describe a build-up of work or, more particularly, of unfulfilled orders. But this is a meaning it has had only since 1932 – was that the year that customer care became the vogue or was it the year that inefficiency first made an appearance? But the word’s origins were much more prosaic – it was used, principally in America and Canada, in the late 17th century to describe the largest log on a fire which was always put to the back. By the 1880s it was used figuratively to depict something stored for later use, just as the back log was the last to burn. (windowthroughtime.wordpresse)
The two assumptions have in common a piece of wood, but its usage and context was totally different. Is there any evidence to support one theory vs the other, or are there other possible credible assumptions to make?