Lambing is when a mother sheep (an ewe) gives birth to lambs. Farmers talk about lambing season — often late winter or spring — and about nights spent in the shed because ewes are lambing. You can use it as the name of that time, or as the action: “She’s lambing tonight.” It is farm language, not city small-talk. Though actually, it is a perfectly ordinary verb on a sheep farm, as everyday there as “the baby is coming” is in a family. It is not about cooking lamb, and it is not a cute way to say “acting like a lamb.”