At it

At \At\, prep. [AS. [ae]t; akin to OHG. az, Goth., OS., & Icel.
   at, Sw. [*a]t, Dan. & L. ad.]
   Primarily, this word expresses the relations of presence,
   nearness in place or time, or direction toward; as, at the
   ninth hour; at the house; to aim at a mark. It is less
   definite than in or on; at the house may be in or near the
   house. From this original import are derived all the various
   uses of at. It expresses: 
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   1. A relation of proximity to, or of presence in or on,
      something; as, at the door; at your shop; at home; at
      school; at hand; at sea and on land.
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   2. The relation of some state or condition; as, at war; at
      peace; at ease; at your service; at fault; at liberty; at
      risk; at disadvantage.
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   3. The relation of some employment or action; occupied with;
      as, at engraving; at husbandry; at play; at work; at meat
      (eating); except at puns.
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   4. The relation of a point or position in a series, or of
      degree, rate, or value; as, with the thermometer at
      80[deg]; goods sold at a cheap price; a country estimated
      at 10,000 square miles; life is short at the longest.
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   5. The relations of time, age, or order; as, at ten o'clock;
      at twenty-one; at once; at first.
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   6. The relations of source, occasion, reason, consequence, or
      effect; as, at the sight; at this news; merry at anything;
      at this declaration; at his command; to demand, require,
      receive, deserve, endure at your hands.
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   7. Relation of direction toward an object or end; as, look at
      it; to point at one; to aim at a mark; to throw, strike,
      shoot, wink, mock, laugh at any one.
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   {At all}, {At home}, {At large}, {At last}, {At length}, {At
   once}, etc. See under {All}, {Home}, {Large}, {Last} (phrase
      and syn.), {Length}, {Once}, etc.

   {At it}, busily or actively engaged.

   {At least}. See {Least} and {However}.

   {At one}. See {At one}, in the Vocabulary.
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   Syn: {In}, {At}.

   Usage: When reference to the interior of any place is made
          prominent in is used. It is used before the names of
          countries and cities (esp. large cities); as, we live
          in America, in New York, in the South. At is commonly
          employed before names of houses, institutions,
          villages, and small places; as, Milton was educated at
          Christ's College; money taken in at the Customhouse; I
          saw him at the jeweler's; we live at Beachville. At
          may be used before the name of a city when it is
          regarded as a mere point of locality. "An English king
          was crowned at Paris." --Macaulay. "Jean Jacques
          Rousseau was born at Geneva, June, 28, 1712." --J.
          Morley. In regard to time, we say at the hour, on the
          day, in the year; as, at 9 o'clock, on the morning of
          July 5th, in the year 1775.
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