Spankable is a coined adjective that means "well suited for spanking" or "nice to spank". It can apply to a person, such as in "she has such a cute bottom—she's really the most spankable girl I know." Note that spankable does not always mean that the person deserves a spanking or is going to get one at all; it can mean "cute enough to spank" in a similar hypothetical manner as "cute enough to eat".
Similar to huggable, spankable can be a physical characteristic, but also character dependent one. A person can also be particularly spankable, for example, not because of their looks but because they react so nicely whenever they are spanked.