12th cent. From Latin tantum, accusative of tantus 'id.'
From Proto-Italic *tānto- "so great." From Proto-Indo-European *teh2-nt-(o-) 'id.,' from demonstrative stem *teh2- (see tal) and *-nt(o)- (a common ending of unclear meaning, see Lowe (2011) for discussion).
Tan (adv.) "so," "also" is an apocopated form.
Portuguese tanto, Catalan tant, French tant, Italian tanto
Oscan etanto "so great," Umbrian etantu 'id.'