'bantamweight' is formed from 'bantam' + 'weight'; 'bantam' (originally a name) came to mean 'small' or 'small and aggressive', and 'weight' here denotes a weight class.

'bantam' originates from the name of the port city 'Bantam' (now Banten) in Java and was used in English from the 17th century for small chickens ('bantams'); the compound 'bantamweight' arose in the late 19th century in boxing to name a lighter weight division and the fighters within it.

Originally referring to a small breed of chicken (and by extension something small and feisty), the term was extended to describe small fighters and then formalized as the name of a boxing weight class; its modern sense centers on the athletic weight division (and, figuratively, small but spirited persons).