Capable of being indicated, suggested, or shown; or able to be ordered or reserved in advance (rare archaic form).
From 'bespeak' plus the '-able' suffix, which forms adjectives meaning 'capable of being' or 'worthy of being.' This form is extremely rare in modern English.
The '-able' suffix is so productive that you can technically add it to almost any verb, which is why 'bespeakable' is grammatically valid but virtually never used—English speakers simply say 'indicative' or 'suggestive' instead.
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