glare at, toil
盻 centers on an intense, hostile gaze: glaring at someone with anger or resentment. The 'toil' sense is a rare extension, possibly from the strain of such a glare or from a separate usage.
盻 combines 目 (eye) with 兮, which likely contributes a phonetic element. The exact historical development is uncertain, but the eye component clearly relates to looking or glaring.
The eye 目 on the left is fixed in a harsh stare, while the right side 兮 looks like a furrowed brow and tense mouth. Together they show a face glaring with anger.
For ケイ, imagine a glaring person shouting 'Kay!' in anger: Kay -> ケイ, and the eye 目 is locked in a fierce glare.