'To be in' definitions:

Definition of 'To be in'

From: GCIDE

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]