tomorrow: [OE] Tomorrow was formed (following the model of today) from the preposition to (here in the sense ‘at, on’) and morgenne, the dative form of Old English morgen ‘morning’, which has evolved into modern English morn and morrow. => morning
mid-13c., to morewe, from Old English to morgenne "on (the) morrow," from to "at, on" (see to) + morgenne, dative of morgen "morning" (see morn, also morrow). As a noun from late 14c. Written as two words until 16c., then as to-morrow until early 20c.