Is it farther or further?
late 14c., from Latin secretus "set apart, withdrawn; hidden, concealed, private," past participle of secernere "to set apart, part, divide; exclude," from se- "without, apart," properly "on one's own" (see se-) + cernere "separate" (see crisis).
As an adjective from late 14c., from French secret, adjective use of noun. Open secret is from 1828. Secret agent first recorded 1715; secret service is from 1737; secret weapon is from 1936.
"to keep secret" (described in OED as "obsolete"), 1590s, from secret (n.). Related: Secreted; secreting.