1
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Phrase(s): on spec
1. using money risked in the hope of profit; on speculation. • He lives by buying and selling houses on spec. • I think it might be an idea to build a few yachts on spec just now. 2. as specified; right on specifications. • This has been built exactly on spec—just as you asked. • It’s important to make sure the design is on spec or the customer will not pay.
McGrawhill's American Idioms And Phrasal Verbs
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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.
American Heritage Idioms