noun
Humble term used to refer to one's own son when speaking to someone outside the family. Not used for other people's sons.
せがれが来月結婚することになりました。
My son is getting married next month.
うちのせがれはまだ大学生です。
My son is still a university student.
noun
young punk; brat
Derogatory or dismissive term for a young man, often implying inexperience or cheekiness. Can be used for someone else's son in a critical tone.
あのせがれは礼儀を知らない。
That brat has no manners.
近所のせがれがうるさくて困る。
The neighborhood punk is so noisy it's a problem.
noun
Vulgar slang; used colloquially to refer to the male organ. Not appropriate in polite conversation.
下品な冗談でせがれの話を出すのはやめてくれ。
Stop bringing up penis jokes with that crude humor.
Rarely used kanji form.
Rarely used kanji form.
息子 is the neutral term for 'son', used for both one's own and others' sons. せがれ is humble when referring to one's own son, and derogatory when referring to someone else's.
坊主 can also be used dismissively for a young boy or kid, but it is broader and less specifically 'son'. せがれ carries a stronger nuance of 'son' or 'young man' with a derogatory edge.
The origin is uncertain. It may derive from 背 (se, 'back' or 'height') + が + れ, but the exact derivation is unclear. The kanji forms are ateji.