A person or thing that complicates or makes something more complex or difficult.
From complicate plus the agent noun suffix -or (from Latin -or). This formation follows the pattern of English nouns describing what something or someone does.
The word 'complicator' is beautifully logical but nearly extinct in real use—we'd say 'the thing that complicates matters' instead. It shows how not every grammatically valid word survives; frequency and feel matter as much as rules.
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