Nowadays, having crisp hair is a sign that you need to switch to a less astringent shampoo. However, when the word crisp entered the English language in the tenth century, it meant—like its Latin source, crispus—that something was curly, and it was often used to describe the curly hair of handsome knights and lovely maidens. It was not until the early sixteenth century that crisp developed its current sense of brittle or crunchy, a shift in meaning caused partly by the sound of the word itself—crisp sounds crisp—and partly because many things, like bacon, not only become curly as they cook but also crisp and crunchy. This crunchy sense of the word crisp led to its being used as a noun in the 1920s when it became, in England, the name of slivers of potatoes fried, salted, and eaten cold—in other words, what North Americans call potato chips. Later still, in the 1960s, crisp gave rise to crisper, a drawer in a refrigerator where vegetables are kept fresh and crunchy.
To rapidly lower the temperature of a substance, specifically a vegetable, like lettuce, with the intention of inducing a state of firmness and brittleness.