'lightweight' originates from English as a compound of 'light' and 'weight'. 'Light' comes from Old English 'lēoht' meaning 'not heavy' (and also 'bright/clear' in other senses), and 'weight' comes from Old English 'weġht' meaning 'heaviness, a weight'.

'light' + 'weight' were joined in Modern English to form the compound 'lightweight'. The use of 'lightweight' as a noun for a boxing weight class and its participants arose in the late 19th century; the figurative sense meaning an insignificant person developed in the early 20th century.

Initially the compound literally described 'little weight' (something not heavy); over time it extended to technical use for weight classes (e.g., boxing) and then metaphorically to mean 'insignificant person' or 'person lacking seriousness/ability'.