That is the *male side* of a hydraulic trailer brake actuator. The *female* side is normally on or, located close to the trailer tongue. It's on a flexible line that allows for trailer/vehicle turns and dips etc. Normally, on the ones I've been associated with, there's two ears on them that correspond with the opposite ears on the other mate. You'd take the female, push it against the male and turn it about 90 degrees to lock them together.
Typically, the small center section of that male part, will travel inward, as the female is coupled to it and then turned and locked. Then, as you apply the tow vehicle brakes, the internal plunger moves the tongue of the female, sending brake pressure to the trailer brakes. That would be similar to what's called *Glad hands* on component hookups on tractor trailer rigs.
It's really, really, REALLY old system and hasn't been around for years. It was a good thought but, never really caught on.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '18 Honda Africa Twin Adventure Sports DCT
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Sophie character, (mini Schnauzer)