A weak (muʿtall) root contains at least one weak radical. “Weak” here describes morphological behavior, not semantic weakness or an invalid Arabic form.
Word / formMeaning / explanation
وَعَدَwāw is the first radical.
قَالَunderlying wāw is the middle radical.
رَمَىunderlying yāʾ is the last radical.
When there is one weak radical, its position yields mithāl, ajwaf, or nāqiṣ. When two radicals are weak, the root is lafīf, subdivided into separated and adjacent patterns. Together these constitute five weak-root patterns.