Things that prevent movement or action; in sports, players who block opponents; in medicine, drugs that inhibit certain functions.
From 'block,' possibly from Middle Dutch or French 'bloc.' Adding '-er' makes it an agent noun (something that blocks). The word has been used in many technical fields.
A lineman is a 'blocker,' an ad blocker is a 'blocker,' even beta blockers are 'blockers'—it's one of those perfect English words where the shape of the word (hard, solid-sounding) matches what it does.
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