weak, pliable, soft, yielding
耎 centers on a lack of rigidity or resistance: something weak, soft, pliable, or yielding. The meanings all describe a state of being easily bent or overcome.
耎 combines 而 and 大, but the exact historical development is uncertain. It may have originally depicted something large (大) that is soft or yielding, but this is speculative.
The top 而 looks like a soft, drooping mustache, and the bottom 大 is a person. Imagine a large person with a weak, pliable mustache that bends easily—just like something soft and yielding.
For ゼン, picture a Zen master who appears weak and yielding but is actually strong: Zen -> ゼン, and the soft mustache on the large person reminds you of that gentle, pliable nature.