Shrewd, a. [Compar. Shrewder; superl. Shrewdest.]
Etymology: [Originally the p. p. of shrew, v.t.]
1. Inclining to shrew; disposing to curse or scold; hence, vicious; malicious; evil; wicked; mischievous; vexatious; rough; unfair; shrewish. [Obs.] Chaucer. [Egypt] hath many shrewd havens, because of the great rocks that ben strong and dangerous to pass by. Sir J. Mandeville. Every of this happy number That have endured shrewd days and nights with us. Shak.
2. Artful; wily; cunning; arch. These women are shrewd tempters with their tongues. Shak.
3. Able or clever in practical affairs; sharp in business; astute; sharp-witted; sagacious; keen; as, a shrewd observer; a shrewd design; a shrewd reply. Professing to despise the ill opinion of mankind creates a shrewd suspicion that we have deserved it. Secker.
Syn. -- Keen; critical; subtle; artful; astute; sagacious; discerning; acute; penetrating. -- Shrewd, Sagacious. One who is shrewd is keen to detect errors, to penetrate disguises, to foresee and guard against the selfishness of others. Shrewd is a word of less dignity than sagacious, which implies a comprehensive as well as penetrating mind, whereas shrewd does not. -- Shrewd"ly, adv. -- Shrewd"ness, n.
Shades of meaning
Fernald has no entry of its own for shrewd; he lists it under astute, sagacious.
Where he writes about it:
We speak of a knowing rascal, meaning cunning or shrewd within a narrow range, but of a knowing horse or dog, in the sense of sagacious, implying that he knows more than could be expected of such an animal. — from astute
Sagacious is a broader and nobler word than shrewd, and not capable of the invidious sense which the latter word often bears; on the other hand, sagacious is less lofty and comprehensive than wise in its full sense, and more limited to matters of direct practical moment. — from sagacious