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 on spec
Done on a speculative basis; with no assurance of profit. For example, We didn't design our house; the builder built it on spec. The use of spec as an abbreviation for speculation dates from the late 1700s.