solid

In 2004, I asked Merriam-Webster the following question. Neil S. Serven replied as follows.

Q: Whenever I heard the word solid, meaning “big favor”—as on Seinfeld, when Kramer would say, “Do me a solid”—I chalked it up as slang. But when one of my professors recently used the word as such, I consulted your collegiate dictionary, but to no avail. Is “solid” colloquial, slang, or Standard English? Also, is my definition correct?

A: The phrase “do me a solid” (meaning “do me a favor”) appears to be quite new to English, and we can find very few sources employing this sense of “solid” outside of the Internet. As far as we can tell, the phrase may have originated (at least in the popular consciousness) with Seinfeld, since we can find no examples of usage that predate that series.

The 1996 book, Flappers 2 Rappers: American Youth Slang, by Tom Dalzell, gives the definition “favor” for “solid” in its chapter on slang influenced by hip-hop and rap culture, but gives no further information or speculation about its origin. None of the other slang dictionaries that we consulted carry the term.