strong
勁 centers on physical or mental strength: being powerful, robust, or vigorous.
勁 combines 巠 (a phonetic component with an associated sense of straight or strong) and 力 (strength), suggesting powerful force.
The left side 巠 looks like a sturdy framework, and the right side 力 is strength. Together they form a strong, unyielding structure.
For ケイ, imagine a strong kayaker paddling with powerful strokes: kayak -> ケイ, and the kayaker's strength matches the kanji's meaning.