stop, halt, previously, already, long ago
已 centers on a boundary in time or action: something that has already happened, stopped, or ended. The meanings 'previously', 'already', and 'long ago' point to a completed past, while 'stop' and 'halt' mark the point where an action ceases.
已 is an ancient pictograph whose original meaning is uncertain. It is often associated with the idea of stopping or completion, but the exact historical development is unclear.
The single curved stroke with a crossbar looks like a barrier or a line drawn to mark a stop. Once you cross that line, the action is already done.
For イ, imagine an eagle (イーグル) that has already stopped flying and perched on a branch: eagle -> イ, and the stop is already complete.