noun
a vast collection of books
A literary yojijukugo describing an enormous number of books, literally 'sweating oxen and a full house'. Used in formal or written contexts to praise a library or collection.
彼の書斎はまさに汗牛充棟の状態だ。
His study is truly a state of 'sweating oxen and a full house'—crammed with books.
This is the headword itself; no direct synonyms are common in modern Japanese. The concept can be expressed plainly as 膨大な蔵書 (huge book collection).
From Chinese 汗牛充棟 (hàn niú chōng dòng), literally 'sweating oxen and a full house', a classical idiom for an immense collection of books.