مُسَرَّجًا
An adjective for which the poem admits more than one proposed lexical interpretation.
The textbook mentions musarrajan because recovering its intended sense requires unusual explanation or etymological interpretation. Merely recognizing the apparent morphological pattern is not enough to establish the contextual meaning.
This illustrates a subtler form of gharaba: the issue need not be a meaningless expression; the difficulty can arise from an obscure semantic relation or specialized poetic use.