إذا حلف لا يأكل لحما فأكل لحم السمك أو الجراد لا يحنث
In the cited usage-based case, an oath not to eat “meat” is not broken by eating fish or locust.
In interpreting an oath, the operative meaning of the expression can depend on recognized ordinary usage, not merely on a biological classification. The classical example contrasts common “meat” with fish or locust in the application of an oath.
It cannot be resolved by saying that all animal flesh must always be treated identically. The legal meaning of the term and relevant customary usage must be established.
This also helps distinguish dalālat al-naṣṣ (a clearly communicated rationale) from cases where customary word meaning determines the scope of a legal statement. The example of an oath not to speak to a person after that person has died turns on the ability of speech to convey meaning to its addressee.