Huh! Well, I learned something myself today! I had never heard this word, ever, but I think the explanations are probably right. At the time Steinbeck was writing this, he was spending a good deal of time with Joseph Campbell, who was then writing his own book, The Hero with a Thousand Faces. Steinbeck, Campbell, and Ed Ricketts (John's marine biologist friend, and inspiration for Doc in Cannery Row) spent long hours discussing myth and sybolism.
There is not much Steinbeck more than myths of the world. It makes perfect sense to me that he would have absorded this idea into The Chrysanthemums.
I really like this question!
As "mejwestut" explained, yonic symbols pertain to femininity and can be shown in literature as containers of some kind. This symbolism is first seen when the story opens: "The high gray-flannel fog of winter closed off the Salinas Valley from the sky and from all the rest of the world. On every side it sat like a lid on the mountains and made of the great valley a closed pot." By comparing the valley where Elisa lives to a "closed pot", it suggests that Elisa's life is one of confinement. Elisa must exist within the narrow limits of her world on the ranch.
Another example of this type of symbolism would be the flower pot that Elisa uses for the chrysanthemums she gives the tinker. Elisa notices later that the tinker kept the pot but threw away the flowers. The flower pot represents the practical view the men have versus the aesthetic view Elisa has. Beauty is important to Elisa, so the flowers would have been what she valued. The flower pot is what the tinker values (representing practicality), so he tosses the flowers and keeps the pot so he can sell it to someone else. Elisa knows she has no one in her life who shares her feelings, making her feel even lonelier.
This is a Sanskrit word. A yonic symbol is a representation of feminitity and reproductive power. Represented in literature by cups, cauldrons, chalices, goblets, wells, circles, hoops, and other containers.