A Raisin in the Sun: Idioms 1 key example

Asking Ruth to convince Mama to invest in a liquor store, Walter uses the idiom "fly-by-night." "Fly-by-night" means untrustworthy or unreliable, referring to someone who would flee in the middle of the night to avoid creditors seeking the money they lent. Hansberry frequently employs idioms, which are a part of the dialect spoken by the characters of the play and that contribute to the play's realism. Moreover, it turns out Walter's proposition is very literally a “fly-by-night” proposition, as Willy flees with the money given to him by Bobo and Walter. The use of this idiom then foreshadows the reveal that Walter has been conned by Willy.

While the play emphasizes the importance of dreams, Walter’s dream is fueled by a desperation that ultimately blinds him. Not all dreams are created equal, but that doesn't mean pursuing them is wrong, as is evidenced by Mama urging the family to support and love Walter during the play's lowest point. In fact, Walter's family supports him despite believing his dream to be flawed throughout the entire play: Ruth, for example, distrusts Walter’s friends but still advocates on his behalf to Mama, and Mama gives Walter the rest of the money to spend as he sees fit despite her misgivings about the liquor store. The idiom then casually reveals precisely what is wrong with Walter's dream, but the play writ large urges the reader to respect Walter and his dream regardless.