Here is a standard definition of dimple: a small depression in the flesh, either one that exists permanently or one that forms in the cheeks when one smiles. That definition is not exactly correct. It is also possible to have a dimple in one's chin.
The author who says that the character cultivated his dimple means that the character understood the dimple was attractive and encouraged it to form. To be realistic and practical, however, the only way the character could have cultivated his dimple would have been to smile a lot. There simply isn't any other way to cultivate a dimple. Dimples are like hair. Either you have them or you don't and there's really nothing you can do about it.
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