| arsenic | (noun) a very poisonous metallic element that has three allotropic forms; arsenic and arsenic compounds are used as herbicides and insecticides and in various alloys; found in arsenopyrite and orpiment and realgar | Synonyms: As, atomic number 33 |
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| (noun) a white powdered poisonous trioxide of arsenic; used in manufacturing glass and as a pesticide (rat poison) and weed killer | Synonyms: arsenic trioxide, arsenous anhydride, arsenous oxide, ratsbane, white arsenic |
| arsenical | (adjective) relating to or containing arsenic | - |
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| (noun) a pesticide or drug containing arsenic | - |
| arsenide | (noun) a compound of arsenic with a more positive element | - |
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| arsenious | (adjective) relating to compounds in which arsenic is trivalent | - |
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| lessening | (noun) a change downward | Synonyms: decrease, drop-off |
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| loosening | (noun) the act of making something less tight | Synonyms: laxation |
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| (noun) an occurrence of control or strength weakening | Synonyms: relaxation, slackening |
| senile | (adjective) mentally or physically infirm with age | Synonyms: doddering, doddery, gaga |
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| senility | (noun) the state of being senile | - |
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| (noun) mental infirmity as a consequence of old age; sometimes shown by foolish infatuations | Synonyms: dotage, second childhood |
| senior | (adjective) older; higher in rank; longer in length of tenure or service | - |
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| (adjective) used of the fourth and final year in United States high school or college | Synonyms: fourth-year |
| (adjective) advanced in years; (`aged' is pronounced as two syllables) | Synonyms: aged, elderly, older |
| (noun) an undergraduate student during the year preceding graduation | - |
| (noun) a person who is older than you are | Synonyms: elder |
| senioritis | (noun) tiredness with the final year of high school | - |
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| seniority | (noun) the property of being long-lived | Synonyms: longevity |
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| (noun) higher rank than that of others especially by reason of longer service | Synonyms: higher rank, higher status, senior status |
| seniti | (noun) 100 seniti equal 1 pa'anga in Tonga | - |
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| worsening | (adjective) changing for the worse | - |
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| (noun) changing something with the result that it becomes worse | - |
| (noun) process of changing to an inferior state | Synonyms: declension, decline in quality, deterioration |