Diminutive means small. A diminutive person is short and small. A diminutive word is a "cute" version of a word or name: for example, "duckling" is a diminutive of "duck" and Billy is a diminutive form of the name William.
A diminutive name or word is formed from another by the addition of a suffix expressing smallness in size: a booklet is a small book, and a dinette is a small version of a dining set. The adjective diminutive descends from Middle English, from Old French, from Late Latin dīminūtīvus, from Latin dēminuere, "to lessen."