The next day, in one word
English uses tomorrow for the day after this one, both as an adverb and as a noun. You can promise to finish a task then, or treat the word itself as the name of that day. In a looser register it also stands for the near future at large, as in talk of the world of tomorrow. That last stretch is comparatively young. The everyday job of pointing at the following calendar day is the older one, and it is the sense the history of the form was built to carry.
A preposition plus a morning
The origin of the word tomorrow is not a loan from French or Latin. It was assembled inside English from two native pieces: the preposition to in an older meaning at or on, and a declined form of the noun that later appears as morrow. Old English already had the phrase tō morgenne for the day that would open with the next dawn. The second element is the dative of morgen, morning: the case the preposition required. American Heritage traces the same path through Middle English to morow. Etymonline records Middle English spellings such as to morewe and tomorwe from the mid-thirteenth century and points those shapes back to the Old English phrase.
The construction is the same one that produced today and tonight. A speaker marked a time of day with to and let that time stand for a whole period. German morgen and Dutch morgen still serve as both morning and the next day, so the English compound is not an isolated trick. Old English could also say on morgne for on the morrow; Bosworth-Toller cites both that phrasing and tō morgenne in the sense come the next day. The two prepositions treat the coming morning as a label for the coming day.
Middle English kept the expression as two words or as loosely joined forms: to morewe, tomorwe, tomorwen. The Oxford English Dictionary treats the later solid word as a compound formed in English from to and morrow, and gives its earliest citation from around 1275, in the poet Laȝamon. That dating describes the Middle English written record of the fused item; the Old English phrase behind it is older still. A rival shape, tomorn, lasted in northern speech long after southern morrow-forms had taken the literary language. Scots still has the morn with the same meaning.
From morgen to morrow
The noun inside the compound is the ordinary Germanic word for morning. Old English morgen, also spelled margen or mergen in some manuscripts, continues Proto-Germanic reconstructions such as murginaz or murganaz. Handbooks disagree on the exact vowels because the family of forms is messy: Gothic maurgins, Old Norse morginn, Old High German morgan, Old Saxon morgan. English kept two short descendants, morn and morrow, and later built morning with the same ing that evening carries. Morrow itself is a clipped Middle English development of the fuller morewen type.
Whether that Germanic noun goes further back is uncertain. Some comparativists link it to a Proto-Indo-European root for flickering or darkening, and they set beside it Lithuanian merkti, to wink, Russian mrak, gloom, and Sanskrit marka, eclipse, as if morning were named from the fading of night. Others treat the word as specifically Germanic and leave the deeper etymology open. Kluge’s German etymological dictionary already called the pre-Germanic shape obscure and preferred a cautious comparison with Slavic words for growing dark rather than a tidy Indo-European story. The safer claim for English stops at the inherited morning-word.
Morphologically the modern spelling hides the old grammar. Once the dative ending wore away, speakers no longer heard the phrase as to plus a declined noun. Hyphenation kept the joint visible for a long time: two words until the sixteenth century, then to-morrow into the early twentieth, then the solid form printers now expect. The hyphen was a spelling habit, not a change in meaning.
When morning became the next day
The semantic step is small and widespread. If you plan something for the morning, and it is already afternoon or night, that morning is the next day. French demain, from Latin de mane, from the morning, follows the same path. Over time the English word stopped needing an actual sunrise in view. It could mark the next calendar day even at breakfast, when the morning in question was a full night away.
As a noun naming the day after this one, the word is recorded from the late fourteenth century. The further sense of the future, especially a near or hoped-for one, is attested by the end of the nineteenth. Idiom added another layer: by 1839, English could describe reckless haste as acting as if there were no tomorrow. Historical dictionaries also note uses now rare or obsolete, among them attaching the word to a stretch other than a day, such as a week or a month, or to the day after some past date rather than after the speaker’s present. Those senses did not survive as the main meaning.
Morrow drifted the other way. Once a common word for morning, and still recognizable in the old greeting good morrow, it is now mostly literary or archaic, while the compound remains ordinary. The fused phrase outlived the independent noun that supplied its second half. Morn survived in poetry and in morning, but it no longer competes with tomorrow as a way of naming the next day.
What German still makes obvious
A German speaker who uses Morgen for both morning and the next day is following the same logic Old English used when it put tō in front of morgenne. English fused the phrase, dropped the case ending, and then dropped the hyphen. What remains is an everyday adverb whose inner pieces are a directional preposition and a dawn. The next day, in this family of languages, began as the next morning.