notice, pay attention to
睬 centers on directing one's visual attention: noticing something or paying attention to it.
睬 combines 目 (eye) with 采, which likely contributes the sound. The eye component points to visual attention.
The eye 目 on the left and 采 on the right suggest an eye actively picking out details: noticing and paying attention.
For サイ, imagine a sigh of relief when someone finally notices you: sigh -> サイ, and your eye catches their attention.
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