Usage patterns
弱気になるto become discouraged/lose confidence
After losing match after match, he became discouraged.
弱気に転じるto turn bearish (of the market)
The market suddenly turned bearish.
弱気を見せるto show weakness/lack of confidence
A leader shouldn't show weakness in front of subordinates.
弱気な発言をするto make a weak-kneed remark
The coach made a fainthearted remark before the game.