E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Stipulate (3 syl.).
The word is generally given from the Latin stipula (a straw), and it is said that a straw was given to the purchaser in sign of a real delivery. Isidore (v. 24) asserts that the two contracting parties broke a straw between them, each taking a moiety, that, by rejoining the parts, they might prove their right to the bargain. With all deference to the Bishop of Seville, his fact seems to belong to limbo-lore. All bargains among the Romans were made by asking a question and replying to it. One said, An stipem vis? the other replied, Stipem volo (Do you require money? I do); the next question and answer were, An dabis? Dabo
(Will you give it? I will); the third question was to the surety, An spondes? to which he replied, Spondeo (Will you be security? I will), and the bargain was made. So that stipulate is compounded of stips-volo (stipulo), and the tale about breaking the straws seems to be concocted to bolster up a wrong etymology.