An irrelevant and powerless guy, also known as a 'flowerman' and shows signs of 'moist' and 'soggy' behaviour.
There are two types of Sidemen:
1) the 'Bupsed guy' who finances a girl's shopping sprees and receives no sex in return.
2) the general wetwipe; often a guy who is emotionally unstable, and isn't rated by anybody -- not even friends.
A Sideman is the male counterpart to a Sidechick.
Chris: What, did Steven beat that yeah?
Tyrone: Nah man, he took her shopping then she ducked.
Chris: Steven is a sideman bruv!