From BlenderWiki

Shear

The Shear transformation applies a shearing on your selection of objects (in Object mode) or object elements (in Edit mode, vertices/edges/control points/…). As the other transform tools, it uses the view space, and is centered on the pivot point: the shear occurs along the view’s x-axis passing through the pivot point. Everything that is “above” this axis (i.e. has a positive y-axis position) will move (shear) in the same direction as your mouse pointer (but always parallel to x-axis). And everything that is “below” this x-axis will move in the opposite direction. The further away from the x-axis is an element, the more it moves.

Note that the hotkey of this tools is CtrlAlt⇧ ShiftS in Object mode, and CtrlS in Edit mode…

Example

TODO: add a short example…