Whatsit means: A thing (used in a vague way to refer to something whose name one cannot recall, or that one is embarrassed to say).. It is mainly Irish English and may not be understood outside that context. It has drug-related associations; worth a calm conversation about which side of the topic your teen is on. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.