Jerkum means: A lost British dialect drink - a rough cider brewed with plums thrown into the mix, made in the West Country and drunk by farm labourers who wanted something stronger than plain apple cider. It was. 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.