She used to diet on any kind of food she could lay her hands on.

The line is a compact joke built on the double meaning of “diet.” Instead of “diet” as a deliberate regimen of restriction, it is used in the older, broader sense of what someone habitually eats—here, comically expanded to mean anything available. The humor comes from the implied contradiction: a person “dieting” is expected to be selective and disciplined, yet this woman’s “diet” is indiscriminate and opportunistic. As a one-liner, it also satirizes fad dieting and self-deception, suggesting that the label of dieting can mask unchanged (or even chaotic) eating habits.