Capable of being colored; appearing plausible or specious; having a superficial appearance of truth or legitimacy without being genuinely valid.
From Middle English, derived from Old French 'colorer,' from Latin 'colorare.' Originally meaning 'capable of being colored,' it evolved to mean 'appearing to be colored with truth or legitimacy.'
The brilliant thing about 'colourable' is that it means both a literal property (can be colored) AND a figurative one (appears true but might not be)—it's an example of how physical properties of color got metaphorically applied to truth and credibility.
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