Why is Lord Hanuman known as “Pavan putra Hanuman”?
It was because, Vayu god had impregnated Anjana through his mind. Hence, Hanuman, although was born from Anjana and Kesari, is also known as Pavan-putra (son of wind):
मनसा अस्मि गतो यत् त्वाम् परिष्वज्य यशस्विनि |
वीर्यवान् बुद्धि संपन्नः पुत्रः तव भविष्यति || [VRM - 4.66.18]
By which reason I embraced you and impregnate myself within you through my mind, by that reason you are impregnated in a super sensory manner, thereby you will beget a valiant son endowed with intellect
The 66th chapter of Kishkinda Kanda in Valmiki Ramayan describes the whole story.
स त्वम् केसरिणः पुत्रः क्षेत्रजो भीम विक्रमः
मारुतस्य औरसः पुत्रः तेजसा च अपि तत् समः
त्वम् हि वायु सुतो वत्स प्लवने च अपि तत् समः [VRM - 4.66.29,30]
Such as you are, you are Kesari's son through his wife, oh, frightful pugilist, and you are the lineal son of Air-god, and even by your gusting you are selfsame to Air-god, and even by your flying also you are his selfsame to Air-god, in all respects
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