'deep-necked' is a compound formed from 'deep' + 'necked'. 'deep' originates from Old English 'deop' (from Proto-Germanic '*daupoz'), where the root meant 'profound, extending far down'; 'necked' comes from 'neck' (Old English 'hnecca') with the adjectival suffix '-ed', meaning 'having a neck or neckline of a specified kind.'

'deep' developed from Old English 'deop' into Middle English 'deep(e)' and modern English 'deep'; 'neck' developed from Old English 'hnecca' to Middle English 'nekke'/'neck' and modern 'neck'; combining them with '-ed' produced descriptive compounds such as 'deep-necked' in Modern English to describe garments or animals.

Initially the elements referred literally to depth ('deep') and the neck itself ('neck'), so early uses could describe an animal with a physically deep neck; over time the compound became commonly used to describe clothing with a low neckline, and that figurative fashion sense is the dominant modern meaning.