bibliothecarian UK /ˌbɪb.li.ə(ʊ).θəˈkɛəɹi.ən/US /ˌbɪb.li.oʊ.θəˈkɛɹi.ən/adjective 1noun 1forms 1 adjective1 •Of or relating to libraries. noun1 •(no longer used) A librarian. formsbibliothecarians