hebetudinous
Displaying mental lethargy or dullness.
His hebetudinous lack of curiosity enraged the instructor.
Where it comes from
From Latin hebetūdin- + -ous, from oblique stem of Latin hebetūdō, from hebes + -tūdō, from hebeō. By surface analysis, hebetude + -ous.
Adapted from the Wiktionary entry hebetudinous (CC BY-SA 4.0).