The meeting started tense, but as the discussion went on, everyone's attitude gradually thawed.

thawed 是动词 thaw 的过去式。thaw 本意是指"解冻、融化"，这里用作比喻意义，表示"态度或关系的缓和、软化"。就像冰雪融化一样，形容紧张的气氛逐渐变得轻松自然。