References for "glad hand" in its common meaning of "effusive but perhaps insincere greeting" seem easy to find , and it's easy to find definitions for the truck-hose-coupler meaning of gladhand ,  but I'm at a loss for finding out the origin of the latter usage (aside from imagining gladhand might derive from a manufacturer's name). Is any evidence available?
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My conjecture is that a "glad-hand coupler" derives from the definition of "glad hand". From Dictionary.com, one definition is:
The Wikipedia entry on semi-trailers explains that a glad-hand coupler (emphasis mine):
If you go "glad-handing", you are making connections that are insincere. I think that because the couplers are attached but easily breakable, they are called "glad-hand connectors" because they are physically "insincere".