noun
excess flesh; flab; surplus fat
Refers to unwanted body fat, often with a negative or self-deprecating nuance. Commonly used in contexts of dieting or body shape.
運動不足でお腹に贅肉がついてしまった。
I've put on flab around my belly from lack of exercise.
彼女は贅肉を落とすためにジムに通っている。
She goes to the gym to lose excess fat.
脂肪 is the neutral, medical/biological term for 'fat', while 贅肉 is a more colloquial, often negative term for unwanted body fat.
贅 (zei, 'luxury' or 'waste') + 肉 (niku, 'flesh'). The kanji 贅 suggests something superfluous or unnecessary, giving the sense of 'waste flesh'.