verbal phrase, 1902, representing casual (American) pronunciation of should have.
Online Etymology Dictionary, © 2010 Douglas Harper
Examples from the Web for shoulda
You shoulda been here the night Pringle had them marching in formation.The Kenzie Report
“You shoulda said,” he said, then turned back to the tour guide.Makers
We shoulda kept you hawg-tied, 'stead o' lettin' you go free!Tom Swift and The Visitor from Planet X
We shoulda been part of the first gang ever to land on the Moon!Space Tug
A'course I shoulda wised you up earlier about Dunham but I thought you were on.Winner Take All