A conclusive presumption is a legal presumption that is deemed to be automatically conclusive and definitive, regardless of what evidence is presented against it. For example, it is a conclusive presumption that a baby cannot commit a felony.
If a judge rules that a certain thing is a conclusive presumption, then the decision is final.
It is usually fairly obvious to understand why a certain thing would be deemed a conclusive presumption. For example, if a man claimed that another man stole his space shuttle that was parked on Jupiter, it would be a conclusive presumption that this wouldn't be true, because it would be impossible for the man to get a private space shuttle to Jupiter in the first place.
Conclusive presumptions help to clear up legal issues quickly.