Commentary for Judges 5:25

But she gave milk to ascertain whether he was alert enough to differentiate between the taste of water and the taste of milk. This is Yonasan's rendition.

He requested water: That is to say - understand the wisdom of Yael: For Sisera requested water, but she gave him milk - as it is written above (Judges 4:19), "and she opened a pouch of milk and gave him to drink." And that was in order to bring him to sleep.

He requested water, she gave him milk: To make him heavier, in order to quickly bring him to sleep - from the tiredness that he [already] had and the milk that he drank - so that he not notice her striking him. And Yonatan translated [it] (in the Targum), "He requested water from her, Sisera the wicked, etc."