Knocker

Device inside a pinball machine that makes a unique sound to indicate that a free game has been won.

How it works: Electricity is applied to a solenoid coil for a split second, creating a momentary electromagnetic field. The magnetism pulls a metal plunger (often with a hard plastic tip) through the coil. After approximately one inch of travel, the tip of the plunger strikes a hard, fixed surface, usually either a metal plate or the wooden wall of the cabinet. A sound occurs at this moment. As soon as the electricity is removed and the magnetic field disappears, the plunger is returned to its original position by means of a spring.

The knocker is also known as the thwacker, and the accompanying sound is sometimes called (you guessed it) a thwack. Knockers on '80s and '90s pinball machines mostly used a metal plate, creating a very loud "thwack" sound, while most earlier knockers simply knock on wood, making a softer "knock".

To complicate matters further, more recent machines (late-'90s and on, presumably) use a "virtual knocker", which simulates the sound of a real thwacker using the pinball machine's digital sound system. The virtual knocker sounds pretty close to the real thing, but allows for the modern convenience of knocker volume control.

The knocker is one of the many kinds of Cornish faery. It is an ugly creature with a hooked nose that stands no more than three feet tall and lives underground in the Cornish tin mines. They are known as knockers because of the knocking sound they made with their picks and shovels as they too mined the tin. A wise Cornish miner would set aside a small portion of his meal to win the favour of the local knockers as it was well known that the knockers knew where the richest veins of tin were located, and would make them known to those whom they favoured.

Some say that they are the spirits of old miners, others that they are the souls of the Jews who crucified Christ condemned to spend the rest of eternity in limbo. Which is why, it is said that on certain days of the year such as Christmas Day and Easter Day they can be heard singing carols in the mines as their penance.

Also a slang synonym for human female breasts, almost always used exclusively in the plural, and usually considered vulgar. The Oxford English Dictionary earliest citation in this sense for "knocker" is from 1941 by the obscure Jack Smiley in Hash House Lingo and reads "Fix the knockers — look at the nice breasts on that woman". Lest the reputability of the term be cast in doubt, the next citation is from Norman Mailer's Naked and Dead in 1949: "Look at the knockers on her, Murray says." It is almost undoubtedly then a very recent addition to the English lexicon, from the 20^th century in this meaning.

The etymology isn't completely straightforward, but I surmise that it must be at least related to the much older meaning of "knocker" as a "looker", i.e. a person, usually a woman, whose beauty is so striking that it actually knocks you. In this sense the OED lists citations as far back as the 1600's. Certainly there have been known more than one set of human female breasts that have had a knockout effect on the opposite sex.

Knock"er (?), n.

One who, or that which, knocks; specifically, an instrument, or kind of hammer, fastened to a door, to be used in seeking for admittance.

Shut, shut the door, good John ! fatigued, I said;
Tie up the knocker; say I'm sick, I'm dead.
Pope.

© Webster 1913

Knock"er (?), n.

1.

A person strikingly handsome, beautiful, or fine; one who wins admiration; a "stunner." [Slang.]

2.

A species of large cockroach, esp. Blabera gigantea, of semitropical America, which is able to produce a loud knocking sound.

© Webster 1913