One who is addicted to crystal meth.
Rachel: That girl is MESSED. UP.
Kelly: Yeah. I went to high school with her- total meth monkey.
A person who abuses crystal meth, resulting in the tightening of the facial skin, which gives the face a monkey-like appearance with sucked-in cheeks, pronounced cheekbones, and outstretched ears. The term possibly originates also from monkey on your back.
His crystal meth habit has turned him into a meth monkey.