It’s a poetical term from Old Possum’s Book of Practical Cats by TS Eliot. That book was the basis for the musical Cats.”
Jellicle cats” are briefly mentioned in T. S. Eliot’s 1933 poem “Five-Finger Exercises”, although they are not described until Eliot’s poem “The Song of the Jellicles”, where Jellicle cats were depicted as commonly nocturnal black and white, scruffy cats.
Specifically, Eliot mentions that they like to gather at an event called the “Jellicle Ball”. The name “Jellicle” comes from an unpublished poem by Eliot entitled “Pollicle Dogs and Jellicle Cats”, where “Pollicle dogs” is a corruption of “poor little dogs” and “Jellicle cats” of “dear little cats”.[3]