'defect' originates from Latin, specifically the past-participial noun 'defectus' (from the verb 'deficere'), where 'de-' meant 'away' and 'ficere' (from 'facere') related to 'make/do' (in practice conveying 'to fail' or 'to be lacking').

'defectus' in Latin passed into Old French (as a noun/verb form), then into Middle English as 'defect', and eventually became the modern English word 'defect' with both noun and verb senses.

Initially, it meant 'a failing or lack' in Latin; over time English developed both the noun sense 'a flaw or shortcoming' and the verb sense 'to abandon or fail (a person, cause, or organization)'.