What does 積ん読 Tsundoku mean?

Tsundoku is buying books and not reading them, or more precisely letting them accumulate. The word is a joke, constructed at the turn of the twentieth century, and the joke is visible in the writing.

The pun

積んでおく (tsunde oku) means to pile something up and leave it there, a completely ordinary phrase. Someone in the Meiji period replaced the final element with 読 (doku), the character for reading, producing something that reads as piling-up-reading, and the resulting form compressed into a single word.

So the construction contains its own commentary: the reading is right there in the word, attached to a verb phrase about leaving things in a heap.

What it does not mean

It does not mean hoarding, and it carries no accusation of pretension. The English framing sometimes turns it into a diagnosis about people who buy books to look clever, which is a different behaviour and a much less sympathetic one.

The Japanese word describes a situation, not a motive. Books were acquired faster than they could be read, which is what happens to anyone who reads and also enters bookshops.

Why English adopted it so readily

Because the experience is universal and English had only unflattering vocabulary for it. A backlog sounds like work. A to-read pile is a description, not a name. Anything else available implies either failure or affectation.

A word that names the pile without judging its owner filled an obvious gap, which is why it spread through book-review culture and library writing rather than through the channels that carried most of this vocabulary.

The unusual route

Nearly every other word on this site arrived through anime, manga or games. This one arrived through people who write about reading, and it is now used fluently by English speakers who could not name a single Japanese author.

It is also, quietly, the best argument against the idea that borrowed Japanese words are exotic ornaments. This one was borrowed because it was useful, by people with no interest in its origin, which is how borrowing normally works.