I cannot guess why it is so, but those who know the least speak the most.

[E non so io indovinare donde ciò proceda, che chi meno sa più ragioni.]

Giovanni della Casa (1503-1556) Florentine poet, author, diplomat, bishop

Galateo: Or, A Treatise on Politeness and Delicacy of Manners [Il Galateo overo de’ costumi], ch. 24 (1558) [tr. Einsenbichler/Bartlett (1986)]
    (Source)

(Source (Italian)). Alternate translations:

Nor can I guess at the cause, (though it is certainly fact) why he that knows the least, should always talk the most.
[tr. Graves (1774)]

I cannot divine how it happens that the man who knows the least is the most argumentative.
[Source]