Perhaps a future of ἐγγυμνάζω , which is used with the accusative of what is being exercised (body, soul, mind) and dative of the skill being practised. But the future middle participle appears as ἐγγυμνασόμενος with s-stem in Plato (see LSJ s.v.)
Alternatively, the headword could be present tense of a compound ἐγγυμνόω (suggested by DW).