switchblade n.

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.

Old English blæd "a leaf," also "a leaf-like part" (of a spade, oar, etc.), from Proto-Germanic *bladaz (source also of Old Frisian bled "leaf," German Blatt, Old Saxon, Danish, Dutch blad, Old Norse blað), from PIE *bhle-to-, suffixed form (past participle) of root *bhel- (3) "to thrive, bloom."

The meaning was extended in Middle English to the broad, flattened bone of the shoulder (c. 1300) and the cutting part of knives and swords (early 14c.). The modern use in reference to grass may be a Middle English revival, by influence of Old French bled "corn, wheat" (11c.), which is perhaps from Germanic. The cognate in German, Blatt, is the general word for "leaf;" Laub is used collectively as "foliage." Old Norse blað was used of herbs and plants, lauf in reference to trees. This also might have been the original distinction in Old English. Compare leaf (n.).

Used of men from 1590s; in later use often a reference to 18c. gallants and dashing rakes, but the original exact sense, and thus signification, is uncertain.