ex ‧ pa ‧ ti ‧ ate /ɪkˈspeɪʃieɪt/ BrE AmE verb
[ Date: 1500-1600 ; Language: Latin ; Origin: exspatiatus , past paticiple of exspatiari 'to wander freely' , from spatium 'space' ]
expatiate on/upon something phrasal verb
formal to speak or write in detail about a particular subject