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).
Their vacation . . . falls in so pat with ours.
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.
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.
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.