帮忙解释一下下面这句话中的“in" 最好是能够还原一下这个句子.One theory,dreamt up in all the spare time enjoyed by the alphabetically disadvantaged,is that the rot sets in early.

帮忙解释一下下面这句话中的“in" 最好是能够还原一下这个句子.
One theory,dreamt up in all the spare time enjoyed by the alphabetically disadvantaged,is that the rot sets in early.