'anti-' originates from Greek, specifically the word 'anti', where it meant 'against'; 'pepsin' originates from Greek, specifically the word 'pepsis' (from 'peptein'), where it meant 'digestion'.

'pepsin' was coined in modern European scientific usage (e.g. French 'pepsine') in the 19th century from Greek roots, and the compound 'anti-pepsin' was formed in scientific/medical contexts by combining the prefix 'anti-' with 'pepsin'.

Initially the root 'pepsin' referred to 'digestion' or the digestive principle; over time 'pepsin' became the name of the specific digestive enzyme, and 'anti-pepsin' has meant an agent acting against that enzyme (this medical/scientific sense has remained stable).