The Collaborative International Dictionary of English v.0.48:

In \In\, adv.
   1. Not out; within; inside. In, the preposition, becomes an
      adverb by omission of its object, leaving it as the
      representative of an adverbial phrase, the context
      indicating what the omitted object is; as, he takes in the
      situation (i. e., he comprehends it in his mind); the
      Republicans were in (i. e., in office); in at one ear and
      out at the other (i. e., in or into the head); his side
      was in (i. e., in the turn at the bat); he came in (i. e.,
      into the house).
      [1913 Webster]

            Their vacation . . . falls in so pat with ours.
                                                  --Lamb.
      [1913 Webster]

   Note: The sails of a vessel are said, in nautical language,
         to be in when they are furled, or when stowed. In
         certain cases in has an adjectival sense; as, the in
         train (i. e., the incoming train); compare up grade,
         down grade, undertow, afterthought, etc.
         [1913 Webster]

   2. (Law) With privilege or possession; -- used to denote a
      holding, possession, or seisin; as, in by descent; in by
      purchase; in of the seisin of her husband. --Burrill.
      [1913 Webster]

   In and in breeding. See under Breeding.

   In and out (Naut.), through and through; -- said of a
      through bolt in a ship's side. --Knight.

   To be in, to be at home; as, Mrs. A. is in.

   To come in. See under Come.
      [1913 Webster]