Fool, equally stupid originally (3)
I believe the answer is:
ass
'fool' is the definition.
(both can mean a stupid person)
'equally stupid originally' is the wordplay.
'equally' becomes 'as' (synonyms).
'originally' says to take the initial letters.
The initial letter of 'stupid' is 's'.
'as'+'s'='ASS'
(Other definitions for ass that I've seen before include "Hardy and sure-footed animal" , "Animal; fool" , "Donkey or foolish person" , "Quadruped; fool" , "Clot" .)