Other forms: deeper, deepest
Deep means going a long way down from the top, or a long way in from the front. Deep water, a deep cut, deep in the forest.
The measurement pattern is worth learning as a set: the pool is two metres deep, and the noun is depth. From the physical sense come a great many others — a deep voice is low, a deep colour is dark and strong, a deep sleep is hard to wake from, and deep thought is serious. The adverb deeply is used for the figurative senses — deeply grateful, deeply worried — while deep is used for the physical: dig deep, deep down. Note deep down, which means really, underneath everything.
1adjectivedeepGoing a long way down from the surface.
“The lake is deep enough to dive into.”
synonyms: profound
2adjectivesoundOf a sound, low in pitch.
“He has a deep voice that carries.”
3adjectivecolourOf a colour, dark and strong.
“The walls are a deep green.”
4adjectiveseriousSerious, or hard to understand.
“That is a deep question for a Monday.”
Too few real examples of this word to estimate which sense is commonest.
deep is a measurement of distance down.
Old English deop goes back to a Proto-Indo-European root meaning deep or hollow, which also gives dip and dive. The figurative uses — a deep voice, a deep thinker — are all English's own extensions of a word that started out purely about distance downwards.
Proto-Indo-European
*dʰewbʰ-
Middle English
dep
English, today
deep
From Middle English dep. From Middle English dep, deep, depe, from Old English dēop (“deep, profound; awful, mysterious; heinous; serious, solemn, earnest; extreme, great”), from Proto-West Germanic deup, from Proto-Germanic deupaz (“deep”), from Proto-Indo-European dʰewbʰ- (“deep”). 2
Etymology from Wiktionary, CC BY-SA