Panzerkin is a nickname for a Balance specced Druid in Moonkin Form with defense oriented gear. The name is a portmanteau of Panzer (a german tank used in World War 2) and Moonkin, so called because of the Moonkin's high armor value makes it a potential tank form, albeit probably not as good as a Feral bear.
Panzerkin can also be used to more specifically refer to a Druid who is off-tanking or tanking in Moonkin form, which can be done with specialized gear and/or in certain circumstances. See Tank, "What is Panzerkin?", and "Lithanials' Guide to Moonkin Tanking v1.1" for more details (see External links below).