To add to the other replies, there's two etymologies that I know of.
The first and most common explanation, is that when a film crew working with an extreme sports athlete would successfully capture a moment on film, they would mail the tape in to the film editor or marketing department - literally send the tape. So when they were doing lots of takes, before rolling the cameras everyone would encourage each other to "send it this time".
(The term "beta" referring to detailed description of a location or technique, came about similarly, as it refers to passing around a literal Betamax tape of another person performing that stunt or rock climb)
The alternate explanation is that it is simply short for "ascend", as in exhorting a rock climber to "ascend it".
The first and most common explanation, is that when a film crew working with an extreme sports athlete would successfully capture a moment on film, they would mail the tape in to the film editor or marketing department - literally send the tape. So when they were doing lots of takes, before rolling the cameras everyone would encourage each other to "send it this time".
(The term "beta" referring to detailed description of a location or technique, came about similarly, as it refers to passing around a literal Betamax tape of another person performing that stunt or rock climb)
The alternate explanation is that it is simply short for "ascend", as in exhorting a rock climber to "ascend it".