historical Each of two heavy beams located on either side of the cylinder in some early steam engines, which are pivoted centrally to oscillate through a small arc and transmit reciprocating motion from the cross head of the pistons to the connecting rods or crankshaft. Frequently attributive.
2In a firearm: a lever that is situated alongside the action and operated by the user, typically to allow the barrel to be opened for loading and locked to the action for firing.
3Generally. A lever situated at the side of something.
Early 19th century; earliest use found in Transactions of the Society of Arts. From side + lever.