"moving backwards and forwards," 1690s, present-participle adjective from reciprocate (v.). Specifically of machines, "having reciprocating parts," by 1822.
Reciprocating engine. A form of engine in which the piston and piston-rod move back and forth in a straight line, absolutely relative to the cylinder, as in oscillating-cylinder engines: in contradistinction to rotary engine. [Century Dictionary, 1895]
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