A variant or archaic form related to embarking or embarkment; the state of being embarked.
From embark + '-ment' suffix. This appears to be an obsolete or highly specialized variant of embarkment, possibly from Middle English or Early Modern English when word-formation rules were less standardized.
Words like 'embarment' are linguistic fossils—they're rarely used today, but they pop up in old texts and remind us that English speakers in the past experimented with many more word variations than we typically use now.
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