The definition of straight is something that is in order, not curly or wavy or is slang for heterosexual or not under the influence of drugs or alcohol.(adjective)
See straight in Webster's New World College Dictionary
Origin: ME streght, pp. of strecchen, to stretch
See straight in American Heritage Dictionary 4
adjective straight·er, straight·est
Origin: Middle English
Origin: , from past participle of strecchen, to stretch; see stretch.
Learn more about straight