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Explanation: I can see that the context is an ironical one.
"Fillings" mean the substance which is used for filling teeth = حشو
and "demented" means a mentally disturbed person = شخص مخرف/خرف .....
The speaker used both "fillings" and "demented" to show ir an ironical way that his cousin is a mentally disturbed person and all his information about stock is merely false fake imagination and lies.
The speaker in an ironical way modified the English idiom which says " "to lie through his teeth" which means "to tell something which is clearly false " into "to tell/say/receive information through his fillings " only to laugh at his cousin and his false information.
I hope I did not go too far away from the correct answer.