即ji2 approach, assume, at once, present. Literally the image of a person approacing the meal with reverence with the servitor (usually) or served (exceptionally).
既ji4 already, since = approach and await meal as a metaphor a symbol for ALREADY [served, eaten] SINCE [dinner, work, etc.]
ji2 and ji4 are closely related etymologically but ji2 is literal and ji4 is metaphoric.