Not sharp or acute, in any sense; qualifying a surgical instrument, the action of the mind, pain, a sound (especially the percussion note), etc. [M.E. dul]
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dull 'dəl adj
1) mentally slow or stupid
2) slow in perception or sensibility
3) lacking sharpness of edge or point <a \dull scalpel>
4) lacking in force, intensity, or acuteness <a \dull pain>
dull vb
dull·ness or dul·ness 'dəl-nəs n
dul·ly 'dəl-ē adv
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(dul) not resonant on percussion.Medical dictionary. 2011.