Sideloading; as it pertains to air cylinders, it is a force applied tangentially to the rod or rod-end tooling.
Typically, air cylinders are linear devices. They are designed to transmit force down the axis of the piston and piston rod.
As a typical double-acting air cylinder operates, compressed air alternately extends and then retracts the rod, by moving the piston from end to end inside the barrel of the cylinder.
When the cylinder is in a retracted position - with minimal extension - any tangential force on the rod tip or rod end tooling, can be more easily absorbed by first:
As pressure is applied sideways on the end of the rod that is retracted, the piston is supported by the inner surface of the air cylinder barrel. The distance between the two bearing points, the rod bearing and the piston itself, ameliorates the side loading.
Sideloading of the cylinder rod is a very common cause of air cylinder failure. One failure mode is leakage at the rod seals. Another is scoring of the rod, creating an abrasive finish that further damages rod seals. Or, enough side load can overcome the pistons ability to move the rod.
Consider ordering an air cylinder with larger-than-standard rod bearings. You can have the cylinder built with a stop collar installed inside the cylinder barrel to ensure that the piston cannot reach the rod end of the cylinder - thus effectively increasing the bearing area. Or, you can have the rod load guided and supported in such a fashion that sideloading is absorbed by the guiding mechanism, and any force from the side is not applied to the cylinder rod at all.
An alignment coupler is a device that connects the end of the rod to the tooling, and can itself be another method of dealing with side load. The alignment coupler self-adjusts to reduce side load as the rod cycles.
Clevis mounts on the rod end connecting to a mating clevis in the tooling will allow some movement of the tooling without side loading the cylinder rod.
Regardless of the method you use to eliminate cylinder rod side loading, failure to do something will mean a much shortened life cycle for any air cylinder.