Sentence:  When Scrooge's nephew laughed in this way—holding his sides, rolling his head, and twisting his face into the most extravagant contortions—Scrooge's niece, by marriage, laughed as heartily as he. And their assembled friends, being not a bit <WSD>behindhand</WSD>, roared out lustily. 
Meaning of the word marked with <WSD> tags:  Inferior, less advanced (compared "with" someone "in" something).