وفي اصطلاح أهل الأصول: هو استدعاء ترك الفعل بالقول ممن هو دونه
In usul, nahy is a verbal demand by one in a superior position that another refrain from an act.
Lā tafʿal (do not do) is the familiar prohibitive form; for example, “do not lie.” Its default legal force here is prohibition, though reliable contextual indicators may establish admonition, guidance, or another intended sense.
When reading such a text, identify who is addressed, the act prohibited, and the context. The form alone does not settle whether a legal transaction or worship performed in breach of the prohibition has legal effect.