c. 1600, in grammar, "the form or number relating to two," from Latin dualis "that contains two; the dual number, duality," from duo "two" (from PIE root *dwo- "two"). General sense of "relating to two, expressing two, composed or consisting of two parts" is from 1650s. Related: Dually.

1590s, "slender riding whip," probably from a Flemish or Low German word akin to Middle Dutch swijch "bough, twig," or swutsche, variant of Low German zwukse "long thin stick, switch," from Germanic *swih- (source also of Old High German zwec "wooden peg," German Zweck "aim, design," originally "peg as a target," Zwick "wooden peg"), perhaps connected with PIE root *swei- (2) "to swing, bend, to turn."

The meaning "device for changing the direction of something or making or breaking a connection" is first recorded 1797. "The peg sense suits the mech(anical) applications" [Weekley]; also compare switchblade. These senses in English might be a direct borrowing from those senses in Continental Germanic languages rather than a continuation of the "pliant wand" sense. The meaning "a change from one to another, a reversal, an exchange, a substitution" is first recorded 1920; extended form switcheroo is by 1933.

1610s, "to strike with a switch," from switch (n.). Related: Switched; switching. The meaning "turn (off or on) with a switch device" is first recorded 1853 of trains on tracks, 1881 of electricity, 1932 of radio or (later) television. Sense of "shift, divert" is from 1860. Meaning "to change one thing for another" is recorded from 1919. Switch-hitter is 1945 in baseball slang; 1956 in the sense of "bisexual person."

also in-line, 1913 of printing, 1921 of engines, 1958 of computers, by 1989 of roller skates; from in + line (n.).

"to bundle up into a pack or package," 1915, from package (n.). Related: Packaged; packaging.

1530s, "the act of packing," from pack (n.) + -age; or from cognate Dutch pakkage "baggage." The main modern sense of "a bundle, a parcel, a quantity pressed or packed together" is attested from 1722. Package deal "transaction agreed to as a whole" is from 1952.

1945, originally CARE package, supplies sent out by "Cooperative for American Remittances to Europe," established 1945 by U.S. private charities to coordinate delivery of aid packages to displaced persons in Europe after World War II and obviously named for the sake of the acronym. The name was repackaged late 1940s to "Cooperative for Assistance and Relief Everywhere," to reflect its expanded mission.

1875, "act of making into a package or packages," from package (n.).

also switch-blade, type of folding pocket knife, 1932, from switch (n.) + blade. So called for the "switch" which is pressed to spring the knife open. Earlier a similar tool was known as an Arkansas toothpick (1837) and a clasp-knife (1755).

also switch-board, "device for making interchangeable connections between many circuits," 1867, from switch (n.) + board (n.1).