Almost always appears with a negative — 不轻言 + verb (放弃 fàngqì 'give up', 承诺 chéngnuò 'promise', 失败 shībài 'failure'). The pattern means 'won't lightly say/do X', i.e. takes X seriously.
register
Literary/written register. In casual speech 随便说 (suíbiàn shuō) 'say casually' covers similar ground.
