"darkmans" means: Obsolete English thieves' cant for nighttime or 'the night', used by 16th and 17th century criminal underworld communities to refer to the d.... This is informal slang, common in casual speech, texting and social media, but not appropriate for school work, applications or professional settings. There is no real cause for concern in itself; it is everyday peer vocabulary. If your child uses it, a light comment about audience and register is usually enough — no need to escalate. Context, more than the word, tells you whether to follow up.