The phrase "letting all hang out" typically means to act uninhibitedly, without any restraint or inhibition. An antonym for this phrase would be to show restraint or to be reserved. People who demonstrate restraint are careful and considerate about their actions and words; they think twice before saying or doing anything, particularly in public. Furthermore, people who exhibit restraint are often more introspective, preferring to keep their emotions bottled up rather than expressing them openly. Conversely, people who "let it all hang out" are open and outgoing, preferring to be noticed and heard. They're often impulsive and unafraid to act on their feelings, even if it means taking risks.