Definition of "fuzzable"

Strings are by far the most common primitive that you will use. Strings are everywhere; usernames, IP addresses, directories, and many more things can be represented by strings. Sulley uses the s_string() directive to denote that the data contained within the primitive is a fuzzable string.

2009, Justin Seitz, Gray Hat Python: Python Programming for Hackers and Reverse Engineers

Sulley works by fuzzing the first fuzzable field to be fuzzed. While it is iterating through all the values it wants to try for that field, all the other fields are untouched and remain at their default value.

2012, Charlie Miller, Dion Blazakis, Dino DaiZovi, iOS Hacker's Handbook, page 172