This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
殸 combines 声 (voice) and 殳 (a hand holding a weapon or tool). The exact historical development is uncertain, but it likely relates to sound or striking.
The left side 声 suggests voice or sound, and the right side 殳 looks like a hand holding something. Imagine a hand striking something to produce a sound, but the exact meaning is unclear.
For ケイ, imagine a mysterious sound (声) made by a hand (殳) that sounds like a sharp 'kay' (ケイ) echoing in an empty room.