たわ言 means 'nonsense' or 'rubbish' and is more negative than 戯れ言, which can be light-hearted.
Etymology
Compound of 戯れ (ざれ, 'playfulness') + 言 (こと, 'word'). The reading ざれ is the 連用形 of the verb 戯れる (ざれる, 'to jest'). The alternate reading たわむれごと uses the 連用形 of 戯れる (たわむれる).