yaせるseru
1.

To experience a reduction in body weight, often due to factors such as poor nutrition or difficult life circumstances, resulting in a thinner appearance.

2.

To undergo a change where one's physical mass decreases, typically indicating a state of being less robust or having less flesh.

3.

To have a diminished capacity for growth or productivity in the context of land, where it can no longer support the flourishing of plants.

Example:

若い頃は痩せてたんだよ。