They’ll come out at the wrong time and appear to the wrong people.
Inappropriate language should never appear in the codebase. Keep it out of your object and method names, comments, test data, and examples. Also keep it out of the commit messages and pull request comments in your source control tools.
These things will bleed out at the most embarrassing time.
You could be doing a code review when a director enters the room, only to see that you’ve named a report generation class runStupidReportForJerksInMarketing(). You can be sure he’ll never forget that. Maybe he’ll find someone who actually doesn’t find the job “stupid”.
Test data can likewise leak out to a production system by accident. Next thing you know, a customer is seeing evidence of cuss words in the system. When the business finds out, they’ll be on the warpath.
Unprofessional behavior like this will scare off clients. If you can’t be a professional, then you should seek a different line of work.