Though Kumar did not name anyone, his remarks riled Tejashwi, who is the son of Lalu Prasad, and he took to Twitter daring the chief minister to swear on an affidavit that his son will never enter politics.
"If Nitish uncle is left with even an iota of morality and integrity, he should sign a sworn affidavit stating that his son will never enter politics," Tejashwi tweeted.
Tejashwi (28) is the younger son of RJD chief Lalu Prasad and often refers Kumar as "chacha" (uncle).
Kumar, who is also the national president of the Janta Dal (United), was addressing a function organised by the party's youth wing here.
"The role of youth in politics is very important. We learnt our ropes while we were students and I feel glad when I see youngsters taking an interest in politics. But looking around, we see so many young leaders who owe their position in politics to their families, basically they are interested in acquiring a post in order to make money. A far cry from our times," he said taking a veiled dig at Tejashwi.
"Care must be taken not to get trapped in the prevailing culture of empty talk. As you all know, there are political figures who excessively use social media and go on tweeting about things they do not even understand. They talk big but have no idea of what they stand for," Kumar said alluding to Tejashwi Yadav.
Continuing to vent ire on the CM, the opposition leader further tweeted, "In a 60-minute speech, Nitish uncle spends 45 minutes speaking about Tejashwi. Thanks to the uncle for his focussed concentration on the nephew. After all, Tejashwi is just a kid."