Guarded means careful about what you let others see. A guarded reply, guarded optimism, guarded about her past.
It describes deliberate restraint rather than secrecy — a guarded answer gives something away but not much. Guarded optimism is the fixed phrase for hope that will not commit itself. In hospital reporting, a guarded condition is a serious one whose outcome is uncertain, which is a defined term rather than a vague one. Note guardedly; and unguarded, meaning off your guard.
1adjectiveguardedCareful about what you let others see.
“She was guarded when reporters asked about her private life.”
synonyms: cautious, reticent
2adjectiveseriousSerious and uncertain, of a medical condition.
“Doctors described the injured driver's condition as guarded.”
guarded is a quality of caution.