Valmiki Ramayana Yuddha Kanda Sarga 28 Shloka 15
Original Shlokaपतितस्यकपेरस्यहनुरेकाशिलातले । किञ्चिद्भिन्नादृढहनोर्हनूमानेषतेनवै ।।6.28.15।।
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शिलातले over the mountain, पतितस्य fallen down, अस्य this, कपे monkey, एका one, हनुः jaw, किञ्चित् a bit, भिन्नातेन slightly broken, दृढहनुः solid jaw, एषः this, हनुमान् Hanuman.
"By falling on the mountain one of the jaws of the monkey got broken. This is the reason that he is known as Hanuman."