A person who makes small, petty criticisms without real knowledge or skill; a minor or incompetent critic.
From critic plus -aster, a diminutive suffix (from Latin -astrum) that adds a meaning of inferiority or smallness, similar to how 'poetaster' means a minor poet.
The suffix -aster is a linguistic put-down: it transforms a respectable word into something contemptuous, so a 'criticaster' is basically a critic so bad that they get a special insult-suffix attached to their job title.
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