c. 1200, "brutish, sensual, debased;" late 14c., "in the manner of a beast," from beast + -ly (1). Weakened in British upper crust use to "awfully, exceedingly" by mid-19c. Beastly drunk is from 1794.
Beastly expresses that which is altogether unworthy of a man, especially that which is filthy and disgusting in conduct or manner of life. Bestial is applied chiefly to that which is carnal, sensual, lascivious: as, bestial vices or appetites. [Century Dictionary]