(as) dull as dishwater (或 ditchwater)

extremely dull

無聊透頂，索然無味

dull the edge of

cause to be less keenly felt; reduce the intensity or effectiveness of

減輕…的感覺；削弱…的強度

she'd have to find something to dull the edges of the pain.

她不得不找東西來緩解疼痛。