weapon_cubemap is a weapon entity available in older Source engine games. It has been stripped out in games running on newer Source engine branches, such as Left 4 Dead 2 and Counter-Strike: Global Offensive for an unknown reason.
Fortunately, some games have this weapon restored through mods:
weapon_cubemapexists in Portal 2, but is missing its model and materials, so these will need to be ported from another game.
This entity gives the player a set of spheres (
models/shadertest/envballs.mdl), each with different reflective surfaces. It is used for checking cubemaps.
The commands to receive the weapon_cubemap (typed into the console) is either
impulse 81 or
These commands are considered cheats, meaning that sv_cheats must be set to 1, or developer mode must be enabled, for them to work.
You can switch to this weapon (after having received it) by typing
bind "X" "impulse 81"; "use weapon_cubemap"will bind it to the X key. (You may need to press it twice the first time you use it in a map.)