noun
walking encyclopedia; walking dictionary
Refers to a person who has extensive knowledge and can answer questions on many topics, like a living reference book.
彼は本当に生き字引だ。歴史の質問なら何でも答えられる。
He's a real walking encyclopedia. He can answer any history question.
物知り is a more general term for a knowledgeable person, while 生き字引 emphasizes the breadth and reference-like quality of the knowledge.
Compound of 生き (living) + 字引 (dictionary). The literal meaning is 'living dictionary', used figuratively for a person of great knowledge.