Able to be alleged or claimed; capable of being asserted as fact.
From 'allege' plus the suffix '-able' (from Latin 'habilis' meaning 'able'). This creates an adjective describing something that can be alleged.
While 'allegeable' isn't common in modern English, it shows how flexible our language is—if something is a verb, we can almost always add '-able' to describe whether it's possible to do that action to something.
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