(informal American) speech or comments that are boastful, insulting, or taunting, usually exchanged between opponents in a competition, to intimidate, demoralize, or humiliate the other;
e.g. The referee ruled that the players stop their trash talk and follow good sportsmanship.
(verb)
to make comments that are boastful, intimidating, or taunting, especially to one's opponent in a competition, in order to intimidate, demoralize, or humiliate them;
e.g. The team captain tried not to trash talk the opposing team.