Something that can be guessed or figured out through inference rather than certain knowledge.
From 'guess' (likely from Old Norse 'giska' meaning to guess) plus the suffix '-able' (from Latin 'habilis' meaning able). The ability to guess something developed as a conceptual pairing in English.
The suffix '-able' is incredibly productive in English, meaning speakers can attach it to almost any verb to create new adjectives—'guessable' follows this pattern so naturally that most English speakers understand it immediately even if they've never heard it before.
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