smooth, oily
膩 centers on a rich, smooth texture that feels oily or greasy, like something coated in fat or oil.
膩 combines the flesh radical ⺼ with 貳, which likely contributes the sound and the idea of something rich or abundant, suggesting fatty or oily flesh.
The flesh radical ⺼ on the left and the abundant-looking 貳 on the right suggest meat that is rich and smooth with fat.
For ジ, imagine a rich, oily dish that makes you say 'Gee, that's smooth!' – 'Gee' cues ジ.