Dull - Vine's Expository Dictionary of New Testament Words
Dull[ A-1,Adjective,G3576, nothros ]
slow, sluggish, indolent, dull" (the etymology is uncertain), is translated "dull" in Hebrews 5:11 (in connection with akoe, "hearing;" lit., "in hearings"); "sluggish," in Hebrews 6:12. See SLOTHFUL, SLUGGISH. In the Sept., Proverbs 22:29. Cp. nothrokardios, "slow of heart" (kardia, "the heart"), Proverbs 12:8.
Note: In Luke 24:25 "slow (of heart)" translates the synonymous word bradus. Of these Trench says (Syn. civ), "Bradus differs from the words with which it is here brought into comparison, in that no moral fault or blame is necessarily involved in it; so far indeed is it from this, that of the three occasions on which it is used in the NT two are in honor; for to be 'slow' to evil things, to rash speaking, or to anger (James 1:19, twice), is a grace, and not the contrary. ... There is a deeper, more inborn sluggishness implied in nothros, and this bound up as it were in the very life, more than in either of the other words of this group." Trench compares and contrasts argos, "idle," but this word is not strictly synonymous with the other two.
[ B-1,Adverb,G917, bareos ]
"heavily, with difficulty" (barus, "heavy"), is used with akouo, "to hear," in Matthew 13:15; Acts 28:27 (from Isaiah 6:10), lit., "to hear heavily, to be dull of hearing." In the Sept., Genesis 31:35 (lit., "bear it not heavily"); Isaiah 6:10.