1580s, "to entangle, become entwined confusedly," also "to untangle, disentangle, unwind" (originally with out), from Dutch ravelen "to tangle, fray," rafelen "to unweave," from rafel "frayed thread," which is of uncertain origin. The seemingly contradictory senses of this word (ravel and unravel are both synonyms and antonyms) might be reconciled by its roots in weaving and sewing: as threads become unwoven, they get tangled. The "entangling" meaning is the "more original" sense according to OED. From 1590s in the figurative sense of "make plain or clear;" 1610s as "make a minute and careful investigation." The intransitive sense, of fabric, "become untwisted or disjointed thread from thread" is by 1610s.
1550s, from scare (v.) + crow (n.). Earliest reference is to a person employed to scare birds. Meaning "figure of straw and old clothes made to resemble a person and set in a grain field or garden" to frighten crows and other birds from the crop is implied by 1580s; hence "gaunt, ridiculous person" (1590s). For the formation, compare daredevil.
An 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). Also fray-boggard (1530s). Middle English had skerel, apparently in the same sense, from skerren "scare."