'askable' is formed in modern English by combining the verb 'ask' with the adjectival suffix '-able' (from Old French/Latin via Old French and Latin suffixes meaning 'capable of').

'ask' originates from Old English 'āscian' / 'ȝescian' (also attested as 'axian' in some dialects) and developed into the Middle English forms 'asken' and finally the modern English 'ask'; the suffix '-able' comes from Old French '-able' (from Latin '-abilis'), and combining them produced 'askable'.

Initially, 'ask' meant 'to seek information or request'; adding '-able' simply produced the sense 'capable of being asked', and that basic meaning has remained stable.