The unnecessary third wheel in a group of 3 people. Tin Dogs are usually insignificant people who spend all their time in the shadow of the others.
Me? I'm their man in Havana; I'm their technical support. I'm... Oh my god... I'm the tin dog.
by Anomonon June 27, 2010
A highly useful but under-appreciated member of a group. Comes from a robotic dog ("K-9") from the "Doctor Who" TV series.
Mickey was instrumental in saving the human race, but he considered himself just a tin dog.
by peakeflix January 05, 2015