A person who diddles; someone who cheats, swindles, or wastes time on trivial matters.
From 'diddle' plus the agent suffix '-er,' which creates a noun meaning 'one who does' the action. This follows the standard English pattern of verb-to-agent-noun conversion.
Agent nouns with '-er' are incredibly productive in English—from any verb you can almost always create a person noun: teach/teacher, run/runner, diddle/diddler—showing how fundamental this pattern is to our language.
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