Dis

SLAM

["Criticize harshly or shut forcefully, both start with this sound."]

The clue "Dis" is short for "diss," meaning to insult or criticize, which can also be called "slam" in slang. The answer plays on the idea of disrespecting or putting someone down, making "slam" a fitting synonym. It’s a straightforward example of slang substitution in crossword wordplay.