He never finished what he was going to say for at that moment something happened. The high-backed chair in front of the fire moved suddenly and there rose up out of it - like a pantomime demon coming up out of a trapdoor the alarming form of Uncle Andrew. They were not in the empty house at all; they were in Digory's house and in the forbidden study! Both children said "O-o-oh" and realized their terrible mistake. They felt they ought to have known all along that they hadn't gone nearly far enough.
I've looked up the dictionary:
out of it  English  Adjective
out of it (comparative more out of it, superlative most out of it)
(idiomatic) Not participating in some trend or group. When my old friends turned up, my wife felt quite out of it. (idiomatic) Disoriented; not thinking clearly. Having the flu all week left me pretty well out of it. (idiomatic) Drunk.
It's not quite right no matter it is talking about the chair or Uncle Andrew.
And the pantomime demon?