'anti-boastful' originates from Greek and Middle English elements, specifically the prefix 'anti-' from Greek 'anti' where 'anti' meant 'against', and 'boastful' built from Middle English 'bosten' (from 'boast') where 'boast' meant 'to brag'.

'anti-boastful' formed by combining the relatively recent productive prefix 'anti-' with the adjective 'boastful' (from Middle English 'bosten'/'boast'), with 'anti-' attached to describe opposition to the attitude expressed by 'boastful'.

Initially, the parts conveyed 'against' + 'bragging'; over time the compound has been used to mean either 'opposed to boasting' (critical stance) or 'not given to boasting' (modest character).