One who ensures or makes something certain; someone providing a guarantee.
From 'ensure' plus '-er', the agentive suffix. While grammatically sound, the word remains rare because 'guarantor' or 'insurer' serves the concept more effectively in modern usage.
English has this habit of creating multiple words for the same concept through different affixes, then abandoning all but one—'ensurer' is perfectly grammatical but lost the competition to 'insurer' in business and 'guarantor' in general speech.
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