Capable of being fitted; suitable or able to be adjusted to fit properly.
From 'fit' plus the suffix '-able' (capable of being), creating an adjective that describes something that can be fitted. Standard English word-formation pattern.
Tailors and engineers constantly use 'fittable' when deciding whether fabric or parts can be adjusted properly—it's a practical word that asks 'can this work?' rather than 'is this perfect?'
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