A wedding under duress, esp when the bride is pregnant; a forced marriage; military wedding(1927+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with shot-gun wedding
An agreement or compromise made through necessity, as in Since neither side won a majority, the coalition government was obviously a shotgun wedding. This expression alludes to a marriage precipitated by a woman's pregnancy, causing her father to point a literal or figurative gun at the responsible man's head. Its figurative use dates from the mid-1900s.