Why do I need to know pleurnicher?
Because you'll want to know if someone accuses you of doing it.
What does it mean?
Pleurnicher is a variant of pleurer, which means ‘crying’, but while the latter implies real sadness, pleurnicher usually means that someone is acting like a big baby.
Defined as ‘crying for no or little reason’ or ‘pretending to cry’, pleurnicher is what Anglophones would call ‘whining’ or ‘whingeing’.
It's not a particularly nice thing to accuse someone of, and it's often used as way of discredit someone's chagrin as unwarranted (sometimes accurately so).
Politicians often use pleurnicher to reject opponents’ complaints. A boyfriend could say it to scoff at his girlfriend's crocodile tears (or vice-versa). Adults could say it if kids are being whiny.
Je n'aime pas les enfants qui pleurnichent tout le temps – I don’t like kids who whine all the time.
Ca fait quatre heures qu’il pleurniche pour avoir une glace – He’s been whining for an ice cream for four hours
You can also change the verb pleurnicher into pleurnicheries, which translates to ‘whinings’ – which doesn’t really make sense in English, but it does in French:
Arrête tes pleurnicheries, je ne supporte pas ça – Stop your whinings, I can’t stand it.
Chialer is another way of saying pleurnicher.