The reflexive pronoun heautou (and its forms heautō, heauton, heautēs, etc.) means of/to/by himself/herself/itself — the subject acting upon or referring back to itself. It appears ~319 times in the NT and is essential for expressing self-denial, self-giving, and mutual submission.
The reflexive pronoun heautou carries immense theological weight because of how the NT uses self-reference language. Christ's self-emptying (kenōsis): Philippians 2:7 — 'he emptied himself (heauton ekenōsen), taking the form of a servant.' Self-denial: Matthew 16:24 — 'Let him deny himself (arnēsasthō heauton).' Mutual submission: Ephesians 5:21 — 'submitting to one another (allēlois), out of reverence for Christ.' The reflexive pronoun marks a revolution in how the NT conceives of the self: not the ego asserting itself, but the self being laid down in service. Paul's 'I no longer live, but Christ lives in me' (Gal. 2:20) is the ultimate statement of transformed selfhood — the old heauton crucified, the new self hidden in Christ.