俱舍論記 25.99

25.99

「諸靈妙德所依止故」者，答。謂諸神靈勝妙功德故名為神，定是彼神所依止故名之為足，神之足故名為神足。

Regarding "Because it is what various marvelous virtues rely upon": this is the reply. This means that all kinds of wondrous and superior virtues of spiritual efficacy are called "spirit" (ṛddhi), and because concentration is what that spiritual efficacy relies upon, it is called "foot" (pāda); being the foot of spiritual efficacy, it is named "basis of spiritual power" (ṛddhipāda).