The exact measure varied from place to place and according to commodity, and though in 19c. in Britain it acquired a precise legal definition, it varied in U.S. from state to state. Used since late 14c. loosely to mean "a large quantity or number." From late 14c. as "a bushel basket." To hide (one's) light under a bushel is from Matthew v.15.
"dry measure of one-quarter bushel," late 13c., pekke, of unknown origin; perhaps connected with Old French pek, picot (13c.), also of unknown origin (Barnhart says these were borrowed from English). Chiefly of oats for horses; original sense may be "allowance" rather than a fixed measure, thus perhaps from peck (v.). Originally not a precise measure and later sometimes used colloquially as "a great deal" (a peck of troubles, etc.).
also crackerjack, "something excellent," 1893, U.S. colloquialism, apparently a fanciful construction, earliest use in reference to racing horses. The caramel-coated popcorn-and-peanuts confection was said to have been introduced at the World's Columbian Exposition (1893). Supposedly a salesman gave it the name when he tasted some and said, "That's a cracker-jack," using the then-popular expression. The name was trademarked 1896. The "Prize in Every Box" was introduced 1912.
"Your brother Bob is traveling, isn't he?"
"Yep. He's with one of the big racing teams. I tell you, he's a cracker-jack! Wins a bushel of diamonds and gold cups every week."
[Life magazine, Aug. 1, 1895]