Manslaughter means killing someone without planning it beforehand. A defendant who's convicted of manslaughter is guilty of a serious crime, though less serious than murder.
Someone who kills another person accidentally or impulsively might be accused of manslaughter, a legal term that describes a killing which is not quite deliberate murder. Manslaughter is extremely serious, although it's usually punished less harshly than murder. Though the word laughter can be seen in manslaughter, it's no laughing matter — and it rhymes with daughter.