伜

segare
offspring (male), son, or, colloquially, penis

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I encountered the term reading some yakuza-esque dialog, and knew right away that it meant 'son'. but it's not the common word for son, which is musuko, 息子.
according to my partner, the use of segare is more common among older men in Japan. it definitely has a rougher, more masculine feel, to the word.
as an aside, both words are used as colloquial euphemisms for penis. apparently, the notion arises from the fact that you can only make sons with your 'son'. as it were.