Ah, interesting - remembering back to Spanish class I think Tomatillo just means "small tomato" in Spanish?
The -illo suffix in colloquial Spanish has a number of uses, it can mean "little" but it can also mean "-like" (as in similar to) so that tomatillo can mean "tomato-like", or "tomatoish" which sounds about right, because the tomatillo, Physalis philadelphica, the key ingredient in fresh and cooked Latin American green sauces, is not actually a true tomato. Although it is related. It is also called husk tomato, jamberry, husk cherry, mexican tomato, or ground cherry.