Dullness, stupidity, vide eg Ovid, Ars Am. 2.361: quis ____ hic, Menelae, fuit? (6)
I believe the answer is:
stupor
(Other definitions for stupor that I've seen before include "State of bewilderment" , "that'll send you to sleep" , "State of dazedness or bemusement" , "lethargic" , "Marginal consciousness" .)