In forestry and timber framing, this refers to the base end of a tree trunk, which is typically wider and denser. In general carpentry, it means the squared-off end of a board where the end grain is exposed. The term dictates orientation in structural work, where posts are often installed with this denser end facing downward to support greater loads.
Heard on the job
Put the butt of that post on the concrete pier so we've the widest section carrying the load.