The story
From straw + man. Random House Webster's Unabridged Dictionary, 1986 passim, shows first known usages for things insubstantial date to 1585-95. Universal Dictionary of the English Language, 1897, Vol 4, p. 4485, notes “man of straw” as “The figure of a man formed of an old suit of clothes stuffed with straw; hence, the mere resemblance of a man; one of no substance or means; an imaginary person.” Compare West Frisian strieman, Dutch strooman, stroman, German Strohmann, Danish stråmand, Swedish stråman, Norwegian stråmann.
Etymology from Wiktionary, CC BY-SA 4.0.