'expectable' originates from Latin, specifically the verb 'exspectare', where 'ex-' meant 'out' and 'spectare' meant 'to look', combined with the Latin-derived suffix '-able' from 'abilis' meaning 'able to be'.

'expectable' developed in modern English from the verb 'expect' (from Middle English 'expecten' < Old French forms < Latin 'exspectare') with the adjectival suffix '-able' to form 'expectable'.

Initially related to the verb sense 'to look out for' or 'await' (from Latin 'exspectare'), but over time it evolved into the adjectival sense 'able to be expected; likely'.