Meaning/Usage: Used when you want to hear the truth
Explanation: "Straight" is referring to straightforward. When people had to tell someone bad news, they would try to find ways to make it sound less bad. To do so, it would require a lot more words and a lot of creativity. However, when people want to hear bad news, they don't want to hear something without additions made by the speaker. Therefore, it was common for the listener to be firm and tell the speaker to speak be straightforward and to speak the truth without adding things.