A weak stain is a man who is virtually useless in every aspect of manhood. Even his jizz is too weak to leave a stain. Also can be used in place of cuck.
A weak streamer is someone who doesn't have a very vital flow when they urinate, or takes a while to get the job done.
Calling someone a weak streamer is a hit on their manhood and is usually not taken lightly since it is a qualitative observation and not simply an insult. Could be consideredfighting words.
"Damn bro, I never took you for a weak streamer, I guess that explains why Eric took your bitch."