لو حلف «لا يشتري رأساً» فهو على ما تعارفه الناس
If a person swears not to buy a 'head', the oath follows what people customarily mean by that expression.
فلا يحنث برأس العصفور والحمامة
Buying the head of a sparrow or pigeon does not violate this oath.
Lexically, *head* can describe the head of any animal. In the commercial setting of the example, however, only certain heads were conventionally bought and sold under that name. Thus the oath is interpreted according to established commercial usage, not every conceivable lexical instance.
The reasoning proceeds from the literal range, to actual custom, and finally to whether the act falls inside that customary sense. This does not render the term meaningless; it identifies the scope intended in this transaction.