Piquant, a.
Etymology: [F., p.pr. of piquer to prick or sting. See Pike.]
Stimulating to the taste; giving zest; tart; sharp; pungent; as, a piquant anecdote. "As piquant to the tongue as salt." Addison. "Piquant railleries." Gov. of Tongue.
Shades of meaning
Fernald has no entry of its own for piquant; he lists it under racy.
Where he writes about it:
Pungent denotes something sharply irritating to the organs of taste or smell, as pepper, vinegar, ammonia; piquant denotes a quality similar in kind to pungent but less in degree, stimulating and agreeable; pungent spices may be deftly compounded into a piquant sauce. — from racy
As applied to literary products, racy refers to that which has a striking, vigorous, pleasing originality; spicy to that which is stimulating to the mental taste, as spice is to the physical; piquant and pungent in their figurative use keep very close to their literal sense. — from racy