spare dick
To feel stupid for doing something that is, in all honesty, stupid.
"I felt like a right spare dick when I dropped a tennar in the park bin and had to hoke around to get it out."
"Yes you did indeed look like a twat"
"Yes you did indeed look like a twat"
A spare dick in a fucking contest
To describe a someone who gives little or no contribution to a task.
"Jenny didn't help at all, she just stood around like a spare dick in a fucking contest"