| arduous | (adjective) difficult to accomplish; demanding considerable mental effort and skill | - |
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| (adjective) characterized by effort to the point of exhaustion; especially physical effort | Synonyms: backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome |
| (adjective) taxing to the utmost; testing powers of endurance | Synonyms: straining, strenuous |
| arduously | (adverb) in an arduous manner | - |
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| arduousness | (noun) extreme effortfulness | Synonyms: strenuousness |
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| assiduous | (adjective) marked by care and persistent effort | Synonyms: sedulous |
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| assiduously | (adverb) with care and persistence | - |
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| assiduousness | (noun) great and constant diligence and attention | Synonyms: assiduity, concentration |
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| deciduous | (adjective) (of plants and shrubs) shedding foliage at the end of the growing season | - |
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| (adjective) (of teeth, antlers, etc.) being shed at the end of a period of growth | - |
| duo | (noun) a musical composition for two performers | Synonyms: duet, duette |
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| (noun) a pair who associate with one another | Synonyms: couple, duet, twosome |
| (noun) two performers or singers who perform together | Synonyms: duet, duette |
| (noun) two items of the same kind | Synonyms: brace, couple, couplet, distich, duad, duet, dyad, pair, span, twain, twosome, yoke |
| duodecimal | (adjective) based on twelve | - |
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| (noun) one part in twelve equal parts | Synonyms: one-twelfth, twelfth, twelfth part |
| duodenal | (adjective) in or relating to the duodenum | - |
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| duodenum | (noun) the part of the small intestine between the stomach and the jejunum | - |
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| duodiode | (noun) an electron tube having two diodes in the same envelope, with either a common cathode or separate cathodes. | - |
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| duologue | (noun) a part of the script in which the speaking roles are limited to two actors | - |
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| (noun) a conversation between two persons | Synonyms: dialog, dialogue |
| duomo | (noun) the principal Christian church building of a bishop's diocese | Synonyms: cathedral |
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| gastroduodenal | (adjective) of or relating to the stomach and the duodenum | - |
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