Serving to fill out or complete; relating to filling a space or gap.
From Latin expletum with -ory suffix (expressing relation or function). The -ory ending is used in words like mandatory, directory, exploratory.
Expletory is so obscure that even most dictionaries don't include it—it's essentially a dead word, but it reveals how English once had more -ory adjectives before the language streamlined and simplified.
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