To defeat someone in an argument; to refute or confute someone using words or logic.
討論会で彼は相手をあっさり言い負かした。
He easily defeated his opponent in the debate.
何度議論しても母を言い負かすことはできない。
No matter how many times I argue, I can't win an argument with my mother.
彼の豊富な知識で相手を言い負かすのは簡単だ。
With his wide knowledge, it's easy for him to defeat an opponent verbally.