walking encyclopedia, a

A person who has a very large and detailed knowledge of a diverse array of facts and who can recite them when asked. Lily's like a walking encyclopedia when it comes to reptiles.

A very knowledgeable person, as in Ask Rob-he's a walking encyclopedia of military history. A similar expression, a walking dictionary, was used by George Chapman in his poem "Tears of Peace" (c. 1600).

An extremely knowledgeable individual. The term likens a person to a huge amalgamation of learning. One of the earliest versions of this cliché occurred in the poem “Tears of Peace” by George Chapman (ca. 1559–1634): “And let a scholar all Earth’s volumes carry, He will be but a walking dictionary.” In Louisa May Alcott’s Little Women (1869) Meg speaks admiringly of a man she considers “a walking encyclopedia.”