Petite is an adjective, borrowed from French, for a woman or girl who is short and slim in a way people often call neat or pretty. It is about a small build, not about being a child, and not about being weak. You meet it in fashion more than in science: “petite sizes” on a clothes rack are cut for shorter women — a higher waist, a shorter sleeve. Though actually, petite on a label does not always mean thin; it mainly means short. In ordinary talk it can sound like catalogue language, or like a compliment. Said with the wrong smile, it can also feel as if the speaker is shrinking a woman into something cute, so tone matters. People almost never call a man petite; they say short, slight, or small. As a noun, a petite can mean a woman of that build, or that clothing size. French petite just means “small” (for a feminine noun); English kept a narrower, more complimentary sense.