1son (colloquial, somewhat coarse; often used by men, can sound self-effacing or rough)
せがれが大学を卒業したと聞いて、親戚中が驚いた。
When they heard my son graduated from university, all the relatives were surprised.
あのせがれは最近反抗的で困る。
That brat of a son has been rebellious lately and it's troubling.
2a derogatory term for a kid or youngster; punk, brat
町中で騒いでいるせがれどもを注意した。
I told off those noisy brats in town.
親が見ていないと、せがれは好き放題する。
When parents aren't watching, brats do whatever they want.
3vulgar slang for the male genitalia (penis)
その言葉は公共の場では下品なので使わない方がいい。
That word is vulgar and it's better not to use it in public.