from Wiktionary, Creative Commons Attribution/Share-Alike License.
superlativeform of agile: most agile.
Sorry, no etymologies found.
But my sense is the agilest confuse speaking about the writing of software at the lowest level of the program activities - that task level in the IMS - while using the term "waterfall" at the programmatic management processes of a program.
"Among the surface studies team she is the agilest of mind if not the most experienced or intelligent.