1590s, "slender riding whip," probably from a Flemish or Low German word akin to Hanoverian swutsche, a variant of Low German zwukse "long thin stick, switch," from Germanic base *swih- (cf. Old High German zwec "wooden peg," German Zweck "aim, design," originally "peg as a target," Zwick "wooden peg"), perhaps connected with PIE root *swei- "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], and these senses may be a direct borrowing from those senses in Continental Germanic languages rather than a continuation of the "pliant wand" sense. The meaning "a change, a reversal, an exchange, a substitution" is first recorded 1920.
1610s, "to strike with a switch," from switch (n.). Related: Switched; switching. The meaning "turn off or on" 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 1930s in baseball slang, 1956 in the sense of "bisexual person."
To become disinterested or less interested: switches off after listening to her for a minute (1921+)
To inform; snitch (1940s+ Underworld)