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insipid implies a lack of taste or flavor and is, hence, figuratively applied to anything that is lifeless, dull, etc. insipid table talk; vapid and flat apply to that which once had, but has since lost, freshness, sharpness, tang, zest, etc. the vapid, or flat, epigrams that had once so delighted him; banal is used of that which is so trite or hackneyed as to seem highly vapid or flat her banal compliments
See insipid in Webster's New World Roget's A-Z Thesaurus II
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