English deep is the ordinary word for something that extends far down from a surface: a well, a wound, a valley, a layer of snow. The same form also covers senses that have nothing to do with vertical space. A voice can be deep, a colour can be deep, a secret can be deep, and a person can be called deep when thought seems to go further than the obvious. As a noun it can mean the sea, an abyss, or the most intense stretch of a season, as in the deep of winter. As an adverb it still means far down or far in.
A native Germanic adjective
The origin of the word deep is native English, not a later loan from French or Latin. Old English already had the adjective dēop, also written dīop, for something with considerable downward extent from a top or surface. From an early date the same adjective could be figurative: profound, mysterious, serious, solemn. The adverb dēope meant far down. The stem also served as a noun for deep water, especially the sea. Middle English kept the word as dep, deep, and depe. The written English record belongs to the Old English period, before the twelfth century.
Germanic relatives make the inheritance plain. Dutch diep, German tief, Old High German tiof, Old Frisian diap, Old Saxon diop, Old Norse djúpr, Danish dyb, Swedish djup, Icelandic djúpur, and Gothic diups all point to a reconstructed Proto-Germanic adjective deupaz, reached in West Germanic as deup. English did not import the idea of depth from a neighbour. It inherited the word with the rest of its West Germanic stock.
An older root, and where handbooks disagree
Behind the Germanic form lies a Proto-Indo-European root reconstructed as dheub- or dʰewbʰ-, with the sense deep or hollow. Handbooks do not all write the same ancestor. Some follow a reconstruction dheu-b- / dheu-p-; others write a form with an aspirated labial. The disagreement is about the exact shape of the pre-Germanic word, not about whether English deep belongs in this family.
Outside Germanic the same root, or a close variant, is usually compared with Lithuanian dubùs (deep, hollow), Welsh dwfn (deep), and an Old Church Slavonic word for bottom or foundation, often cited as duno. Old Irish domun (world) is sometimes placed here as well, through a proposed chain from bottom to foundation to earth to world. That last path is a scholarly proposal rather than a settled fact. Albanian det (sea) appears as a cognate in some dictionaries; that link is less secure than the tight Germanic set.
The English adjective itself is morphologically simple. It has no prefix and no borrowed suffix. What it does have is a cluster of relatives formed on the same Germanic stem. Dip continues Old English dyppan, a verb of immersing. Dive continues Old English dūfan (to sink, dive) and a related weak verb dȳfan. Depth is a later noun built with the abstract ending -th, the same pattern as length and width. In other Germanic languages a related verb daupjan meant to dip and became the ordinary word for baptizing, as in German taufen. English kept baptize from church vocabulary and did not make deep carry that ritual sense, but the family connection is still visible in dip.
How the senses spread
Physical downward extent was already joined, in Old English, by figurative uses: solemn, mysterious, extreme. By about 1200 the adjective could describe a person of penetrating mind. In the early fourteenth century it could mean extensive in any direction analogous to downward, as when a cupboard is measured from the front rather than from the top. Late in that century it applied to sound that is low in pitch and to colour that is intense rather than pale. From the mid-fourteenth century the noun covered that which is of great depth in general, not only the sea. From the 1560s it could mark involvement: being far advanced in debt or trouble.
Later English kept the old spatial picture in idioms. Figurative deep waters, for a situation beyond comfort or safety, is recorded from the 1530s. Go off the deep end, recorded by 1921, probably takes its image from the deep end of a swimming pool, where a person on the surface can no longer touch bottom. Deep pocket as a figure for wealth is much later, from 1951. An older, now rare, sense used the adjective of roads that were muddy or heavy going.
Sports and anatomy added still more mappings of the same idea: a ball hit to deep right field, a defender playing deep, a structure lying deep to another in the body. Those uses are extensions of distance from a reference surface, not a new word.
A cousin that still means hollow
A useful way to hear the age of the English form is to set it beside a Baltic cousin rather than a modern synonym. Lithuanian dubùs still pairs deep with hollow, the same pair of senses reconstructed for the older root. Welsh dwfn keeps the spatial meaning without the later English overlay of pitch, colour, and intellect. German tief and Dutch diep still do everyday work that Old English dēop already did. The English adjective never needed to be reinvented. It has been measuring distance from a surface, and then distance from the obvious, since before the Norman Conquest.