I was told this by one of my Sanskrit teachers back in school.
The word ‘Manoj’ is a ‘samaasa‘ (compound word) in Sanskrit. (To be specific, it is Avayeebhaava samaasa).
It derives from the explanation: ‘manasi jaatah iti Manojah‘, meaning “he is born in the mind, hence he is ‘Manoj’”. When expanded:
Manasi = in the mind
jaatah = born
iti = therefore/that’s why/hence
Manojah = he is Manoj
It is used often as a reference to Kaamdev (the God of Love, in Hindu mythology, who was burned to ashes by Lord Shiva’s third eye). It is said that Lord Kaama was born in the mind (of Lord Brahma, the creator).
Thanks to this post that reminded me of it.