What does Genesis 26:8 mean?
Isaac has followed his father's example. Both had beautiful wives. In two separate instances, Abraham feared that Sarah's beauty would lead someone to kill him to get her. So, he asked her to lie for him and say she was his sister (Genesis 12:10–20; 20:1–18). In both of those cases, the lie backfired. Both times, a local ruler believed their story and took Sarah for a wife. And in both cases, God brought judgement on the household of the ruler in order to protect and deliver Sarah.In Isaac's case, it seems, a long time passed without anyone making a play for Rebekah. Perhaps the couple forgot to continue to pretend to be siblings or perhaps Abimelech had an unusually good vantage point for observing them. In any case, he saw Isaac and Rebekah acting in a manner which convinced him, absolutely, that they were married. The Hebrew term used here is based on the word tsachaq, which can mean "laughing" or "sporting" or "caressing," depending on the translation. In other words, they were involved in some type of intimate activity which would not be normal for siblings.
As the next verse reveals, that was all Abimelech needed to see to understand that Isaac had lied about his wife.