Missouri does not specifically mention "nunchucks" in their weapons offenses provisions (Missouri Revised Statutes § 571-010
The only weapon which would be similar is a "blackjack", which is defined as follows:"Blackjack", any instrument that is designed or adapted for the purpose of stunning or inflicting physical injury by striking a person, and which is readily capable of lethal use
(Missouri Revised Statutes § 571-010
A "nunchuck" would probably fall within the definition of a "blackjack."
However, the Missouri statutes do not make it a crime to simply own a "blackjack" (see Missouri Revised Statutes § 571.020
Missouri only makes it a crime to carry a concealed "blackjack" upon his person. Specifically, Missouri Revised Statutes § 571-030.1(1)
Section 571-030 Unlawful use of weapons--exceptions--peUnlawful use of weapons--exceptions--penalties.
571.030. 1. A person commits the crime of unlawful use of weapons if he or she knowingly:
(1) Carries concealed upon or about his or her person a knife, a firearm, a blackjack or any other weapon readily capable of lethal use;
So, it would only be illegal if carry a concealed nunchuck on your person.
I hope this answers your question.