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.
Romance: Western Vulgar Latin: Portuguese tanto, Catalan tant, French tant, Italian tanto
Italic: Oscan etanto "so great," Umbrian etantu 'id.'