Is it ensure, insure, or assure?
1550s, from scare (v.) + crow (n.). Earliest reference is to a person employed to scare birds. Meaning "device of straw and cloth in grotesque resemblance of a man, set up in a grain field or garden to frighten crows," is implied by 1580s; hence "gaunt, ridiculous person" (1590s). The older name for such a thing was shewel. Shoy-hoy apparently is another old word for a straw-stuffed scarecrow (Cobbett began using it as a political insult in 1819 and others picked it up; OED defines it as "one who scares away birds from a sown field," and says it is imitative of their cry).
device posted on cultivated ground to deter birds or other animals from eating or otherwise disturbing seeds, shoots, and fruit; its name derives from its use against the crow. The scarecrow of popular tradition is a mannequin stuffed with straw; free-hanging, often reflective parts movable by the wind are commonly attached to increase effectiveness. A scarecrow outfitted in clothes previously worn by a hunter who has fired on the flock is regarded by some as especially efficacious. A common variant is the effigy of a predator (e.g., an owl or a snake).