セキ, シャク, むかし
once upon a time, antiquity, old times
JLPT3 G3 S8 F1197
The relevant oracle bone form of this character is 日 sun/day + a representation of one object layered on another → passage of days/time (likened to a figurative piling up; compare 昨 and 時) → *antiquity*; (previous) *night*.
[Full Kanji Etymology List]