clabber

Sour or curdled milk.

Even butter had proved beyond her means, for the milk she had tried to churn had never firmed up beyond the consistency of runny clabber.

Wet clay or mud.

To sour or curdle.

They always had more milk than they needed and often entire buckets would clabber and one of her brothers would carry it out to the bunkhouse for the vaqueros.