'arsenical' originates from English, formed from the noun 'arsenic' + the adjectival suffix '-al'; 'arsenic' itself comes from Latin 'arsenicum' via Greek 'arsenikon' (ἀρσενικόν), where 'arsenikon' referred to the mineral (orpiment) and carried a sense of 'potent' or 'male'.

'arsenic' passed from Greek 'arsenikon' into Latin as 'arsenicum' and into Medieval and Middle English as 'arsenic'; the adjectival form 'arsenical' was created in English by adding the suffix '-al' to denote 'of or relating to arsenic'.

Initially the root terms referred specifically to the mineral (orpiment) and its strong properties; over time 'arsenical' has come to mean broadly 'containing or relating to arsenic' and, by extension, 'poisonous or having arsenic-like toxic properties'.