fat, grease, growing fat
腴 centers on fatty tissue: the substance of fat or grease, and the process of becoming fat or plump.
腴 combines the flesh radical ⺼ with 臾, which likely contributes a phonetic role. The exact historical development of 臾 is uncertain, but the character is associated with fat or fleshiness.
The flesh radical ⺼ beside 臾 suggests a body part. Picture a plump, fleshy area on the body that stores fat and grease.
For ユ, imagine you touching a greasy surface: you -> ユ, and your finger comes away coated in fat.