opaque

opaque adjective addlebrained, addleheaded, addlepated, ambiguous, asinine, benighted, bewildering, birdbrained, blind, blockish, boeotian, brainless, cloddish, clouded, cloudy, concealed, confused, confusing, cryptic, dark, dense, difficult, difficult to comprehend, difficult to understand, dim, dimwitted, doltish, dull, dull-witted, duncelike, duncical, enigmatic, fatuitous, fatuous, foggy, hard to comprehend, hard to understand, hazy, ignorant, ill-defined, imbecilic, impenetrable, imperspicuous, impervious, incomprehensible, inscrutable, lacking clarity, loutish, lumpish, misty, moronic, muddleheaded, muddy, nebulous, nescient, nontranslucent, oafish, obfuscated, obscure, purblind, puzzling, recondite, senseless, shadowy, simple, simple-minded, slow in understanding, stolid, thick, thick-headed, thick-witted, turbid, unclear, uncomprehending, unenlightened, unfathomable, unintelligent, unintelligible, unplain, unreasoning, vacuous, vague, witless

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See also: impervious, inapprehensible, indefinable, indefinite, obtuse