Brisk, a.
Etymology: [Cf. W. brysg, fr. brys haste, Gael. briosg quick, lively, Ir. broisg a start, leap, jerk.]
1. Full of liveliness and activity; characterized by quickness of motion or action; lively; spirited; quick. Cheerily, boys; be brick awhile. Shak. Brick toil alternating with ready ease. Wordworth.
2. Full of spirit of life; effervescas, brick cider.
Syn. -- Active; lively; agile; alert; nimble; quick; sprightly; vivacious; gay; spirited; animated.
1 other Webster entry under this spelling
Brisk, v. t. & i. [imp. & p. p. Bricked; p. pr. & vb. n. Bricking.]
To make or become lively; to enliven; to animate; to take, or cause to take, an erect or bold attitude; -- usually with up.
Shades of meaning
Fernald has no entry of its own for brisk; he lists it under active, alert, alive, nimble.
Where he writes about it:
Active refers to both quickness and constancy of action; in the former sense it is allied with agile, alert, brisk, etc.; in the latter, with busy, diligent, industrious. — from active
The sense of brisk, nimble is the secondary and now less common signification of alert. — from alert