An auto-antonym is a word with multiple meanings (senses) of which one is the reverse of another.
So apparently the kanji meaning of this word is "previous" or "past". This seems to be tied to the Chinese origin and its on'yomi reading (せん), maybe?
The vocabulary meaning of the standalone word is "ahead" or "future". This meaning may be connected to the Japanese influence on the word and its kun'yomi reading (さき).
Is it true that this kanji (先) can refer to both future events and past ones? How did it evolve to be this way and what has the original chinese meaning to do with it?
UPDATE: So, this kanji means "prior" or "first" in Chinese according to this source. This further validates that it originally was connected to the concept of "past" and the "future" meaning came later and was introduced solely in Japan.
This still does not fully answer any questions.
Also my question are tangentially related to this exchange. But my questions have not been satisfactorily answered by that question and the core differs from what I am interested in.