A person who smears or daubs something onto a surface.
From besmear plus the agent noun suffix -er (Old English -ere), which creates a noun meaning 'one who does' the action. This suffix has been used in English for over a thousand years to name people by their actions.
Agent nouns like 'besmearer' are rare in modern English, but they're incredibly productive in historical texts—you'll find 'befuddlers,' 'bedazzlers,' and 'begrudgers' in older writing. It's a reminder that English once had more flexibility in creating job titles from verbs.
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