Valmiki Ramayana Kishkindha Kanda Sarga 66 Shloka 24
Original Shlokaतदा शैलाग्रशिखरे वामो हनुरभज्यत। ततो हि नामधेयं ते हनुमानिति कीर्त्यते।।4.66.24।।
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तदा then, शैलाग्रशिखरे on the edge of the mountain peak, वामः left, हनुः chin, अभज्यत was broken, ततः then, ते your, नामधेयम् named, हनुमानिति Hanuman, कीर्त्यते wellknown.
'You fell on the edge of the mountain peak and your left chin got broken. Since then you are widely known as Hanuman (one who has a wounded chin).