previously, already, long ago
既 centers on something that has already happened or been completed: previously, already, or long ago. It marks a state that is finished and now taken as a given.
既 originally depicted a person turning away from a food vessel, suggesting the idea of having finished eating. The modern form retains the right-side component 旡, which is related to this turning-away image, while the left side is a simplified vessel shape.
The left side looks like a container, and the right side 旡 suggests someone turning away. Imagine a person who has already finished their meal and turns away from the empty bowl: the action is done, already in the past.
For キ, imagine a key that has already been used to lock a door: key -> キ, and the door is already locked, just as 既 marks something already completed.
already
existing
married
established; existing; accomplished; accepted; completed
(already) known; established
already published
established; fixed; prearranged; predetermined
previously presented; already shown; previously posed (exam question); aforementioned
vested; already acquired
total eclipse; totality
ready-made; off the shelf
the past
established; existing
already finished (action); already accomplished
previous report; previous announcement
total eclipse (solar or lunar); totality
illness that one had in the past; previous illness; medical history; anamnesis
one particle irreducible
in the meantime; meanwhile
falling suddenly