Tilting at Windmills
Finding the etymology of figurative expressions can be difficult . I was able to track down some interesting pages with various expression etymologies, but no "tilting at windmills."
It was easy enough to learn that the expression comes from the novel Don Quixote who ineffectually attacked windmills, being under the illusion that they were giants.
But why tilting? Fortunately, the OED supplied the critical detail that made it all clear: tilting is jousting.
tilting,vbl.n.1
1. a. The action of TILT v.1 in sense 5; charging on horseback with a lance against an opponent, or a mark; justing [ jousting ].
1610 HOLLAND Camden's Brit. (1637) 250 Having unhappily slaine his onely sonne while he trained him at Tilting. 1617 MORYSON Itin. I. 190 On the right hand as you come in..is a place for Tylting, called Tournelles. 1730 A. GORDON Maffei's Amphith. 250 The Armour People put on at tilting with Lances. 1893 K. A. SANBORN Truthf. Woman in S. California 172 The tournament is exciting, where skilful riders try tilting at rings, trying to take as many rings as possible on lance while galloping by.
c. transf. and fig.
1668 HOWE Bless. Righteous (1825) 175 A perpetual hostility, a very tilting at his cross. 1878 STEVENSON Edinburgh (1889) 2 Perpetual tilting against squalls.
joust, just, v.
3. spec. To engage in a joust or tournament; to run at tilt with lances on horseback.
13.. Guy Warw. (A.) 872 Oaines sir Gij er com Gayer, To iuste wi him he drou him ner. c1386 CHAUCER Knt.'s T. 1628 Al that Monday Iusten they and daunce. 1390 GOWER Conf. III. 63 To se..The lusti folk iouste and tourneie. 1485 CAXTON Chas. Gt. 41 Sende to Iuste ayenst me somme of thy barons. c1489 —— Sonnes of Aymon i. 32 They Iousted moche worthyly but Reynawde iousted beste vpon his horse bayarde. a1548 HALL Chron., Hen. VIII 85 The kyng..in his owne person Iusted to all comers. 1608 SHAKES. Per. II. i. 116 There are Princes and Knights come from all partes of the world to Iust and Turney for her loue. 1755 JOHNSON, To joust and to just. 1773-83 OOLE Orl. Fur. XXVI. 524 Every chief..He call'd to joust, and dar'd them to the field. 1805 WORDSW. Prelude IX. 455 Methought I saw a pair of knights Joust underneath the trees. 1825 MACAULAY Ess., Milton (1851) I. 15 Knights, who vowed to joust without helmet or shield. 1868 FREEMAN Norm. Conq. II. viii. 261 Not justing with his lance as in a mimic tourney.
Source: Oxford English Dictionary
What a wonderful expression—tilting at windmills—but that reaction is to be expected from someone who will reply, "Cottleston pie."