Perhaps the earliest and best-known use of this grammatically incorrect phraseology can be found in the 1982 movie, Rocky III, screenplay by Sylvester Stallone. In a cherished and oft-quoted moment from the movie, Mickey, Rocky’s manager and trainer, tells him, “You did good kid, ya did real good.”
The usage stuck, and it has since been used in some 20 movies. To this day, for better or worse, our use of the English language has mimicked a fictional, weary, punch-drunk boxing manager.
“Good” (in addition to being a noun, as in “good versus evil”) is an adjective. An adjective modifies a noun. An example of the correct usage is, “Good grief, Charlie Brown.” Here “good” is the adjective and it modifies “grief,” the noun.
In contrast, “well” is an adverb. It modifies a verb as in, “Well done, son.” “Done” is the verb. “Well,” which modifies “done,” is the adverb. Unfortunately, not many people think to congratulate you on a job “well done” any more. Instead, they say, “You did good.”
When someone says, “You did good,” are they quoting a heartwarming moment from Rocky III—or do they truly believe that they are constructing a grammatically correct sentence?
If “You did good" is shorthand for, “You struck a blow for good,” then the use of “good” is in fact correct. And maybe this classic understanding of “good,” rephrased by Rocky’s small town manager, led to its modern usage.