Lassi is what is left after butter is churned out of curd. It is not a drink invented for a menu; it is the by-product of making ghee, and a farming household produced it daily in quantity.
It is drunk salted and with cumin in the fields, and sweet in the towns. The sweet version with cream on top, sold in Amritsar and Patiala in steel tumblers the size of a bucket, is a commercial development.
The sequence is milk, boiled and set with a starter overnight, churned in the morning with a madhani, butter lifted off, butter clarified into ghee, and lassi drunk.
A household running two buffaloes produced more dairy fat than a modern nutritionist would sanction and burned it walking behind a plough.
Chaach, dahi, paneer, khoya, kheer, and the ghee that goes into everything, all come off the same animal on the same morning. The Punjabi diet is a dairy diet with grain attached.
The madhani is a wooden staff turned inside a clay or brass pot by a rope pulled hand over hand, and in a house with animals it ran before first light. The sound of it is one of the things people who left villages say they remember. The electric churn that replaced it does the same work in four minutes and makes none.
What comes off after the fat is taken is almost fatless, which is why it was never sold and why a house gave it away without thinking about it. Anyone at the door in the hot months got a glass and it cost the household nothing at all.
The whole cycle assumes an animal in the house. A household that buys milk in a packet has no lassi in this sense, and that shift has happened across urban Punjab in one generation.