Japanese (slang), 19th century
Acquiring reading materials and letting them pile up without reading them. It combines elements of tsunde-oku (積んでおく, to pile things up) and dokusho (読書, reading books). As currently written, the word combines the characters for “pile up” (積) and the character for “read” (読).
Example: My vast tsundoku looks overwhelming, but it’s still comforting to me.
Image: Ronnie Filyaw – Whomp!