Murḍiʿ is an active participle from Form IV arḍaʿa, yurḍiʿu. Applied to a woman, murḍiʿ can describe her standing as a nursing mother, while murḍiʿah may emphasize the actual activity of suckling. Usage and the lexicographers’ explanations determine the nuance.
Together, ḥublā and murḍiʿ evoke women with substantial obligations of their own. The next phrase, fa-alhaytuhā, therefore represents his boast that he drew the woman’s attention away.
The grammatical lesson is that an adjective describing a woman does not always require a visible final tāʾ marbūṭah; some feminine referents take forms without it.