What does «sí» mean in Spanish?

Reflexive form of the third person personal pronoun for both genders and numbers; it always takes a preposition. When the preposition is con, we say consigo.
(De por sí) Each item separately.
(De sí) Yours.
(Para sí) Mentally or without saying it to another. This idiom also applies to the pronouns mi and ti.
(Por sí y ante sí) On its own deliberation and without anyone else's input.
(Sobre sí) With attention and caution.
(Sobre sí) With fortitude and haughtiness.