From gumnos; to practise naked (in the games), i.e. Train (figuratively) -- exercise.
see GREEK gumnos
1. properly, to exercise naked (in the palaestra).
2. to exercise vigorously, in any way, either the body or the mind: ἑαυτόν πρός εὐσέβειαν, of one who strives earnestly to become godly, 1 Timothy 4:7; γεγυμνασμενος exercised, Hebrews 5:14; Hebrews 12:11; καρδίαν γεγυμνασμένην πλεονεξίας (Rec. πλεονεξίαις), a soul that covetousness or the love of gain has trained in its crafty ways, 2 Peter 2:14; cf. Winer's Grammar, § 30, 4.