"Lucy in the hands of Sam. Fortunately, he has a neighbor, Annie, who agrees to be Lucy's godmother. Lucy is as precocious as Sam is backward. Sam loves to read easy books for Lucy."
The above is a movie "I Am Sam" review from a newspaper. I don't understand the red sentence.
Does it mean that she is premature while her father is immature?
"A is as B as C is D" is way of saying that A is (very) B while C is (very) D. g. in knowledge or behaviour) for one's age.
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"A is as B as C is D" is way of saying that A is (very) B while C is (very) D.
"precocious", used of children, means very advanced (e.g. in knowledge or behaviour) for one's age.
"backward" means having difficulties in learning or understanding (not a politically correct word nowadays).
A few more comments.
"Lucy is in the hands of Sam."Or 'in the custody of' Sam.
Does it mean that she is premature while her father is immature? The opposite of 'immature' in this context is 'mature'. For children, 'premature' usually means 'born too early', eg a premature baby.