Wisp of Straw
(A). Sign of danger. Often hung under the arch of a bridge undergoing repairs, to warn watermen; sometimes in streets to warn passengers that the roof of a house is under repair. The Romans used to twist straw round the horns of a tossing ox or bull, to warn passers-by to beware, hence the phrase foenum habet in cornu, the man is crochety or dangerous. The reason why straw (or hay) is used is because it is readily come-at-able, cheap, and easily wisped into a bundle visible some long way off.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894