To make something dirty or polluted. The factory's waste fouled the river, making it unsafe for swimming.
To commit a violation of the rules in a game or sport. The player was penalized for fouling his opponent during the match.
To spoil or ruin something. The unexpected rainstorm fouled our plans for a picnic.
To clog or obstruct something, especially a pipe or drain. The kitchen sink was fouled by grease and needed to be cleaned.