What is actually being doubled when someone has to "double back"?
I have frequently heard this phrase and used it myself when I've gone in a wrong direction either physically or at work metaphorically. However, I wonder why the phrase is double back, since once you realize the mistake that you have made, you only go back the way you came once, not twice or double the distance.
Solution 1:
It is figurative.
If you take a rope, and fold it in half so the loose ends are together and your load/friend is hanging from the other, you have doubled the rope.
Walking out to some point and then walking back along the same route is like walking along a doubled rope, and it draws a contrast with a round trip which may have different routes for the outward leg and the return.
Solution 2:
Doubled can mean bent back upon itself. When it is used in this sense, it is often paired with up, down, back, and backward. Hence when your path turns back upon itself you can say that you double back. For examples, see this excerpt from A New English Dictionary on Historical Principles:
8. To bend (a piece of cloth, paper, etc.) over, so as to bring the two parts into contact parallel; to fold; to bend (the body, etc.) so as to bring distant parts into proximity; to close, clench (the hand or fist). Often with up.
(In quot. 1589, to close (the ears).)c 1430 Two Cookery Bks. 39 Take a pese of fayre Canneuas, and doble it. 1589 PUTTENHAMEng. Poesie III. xxiii. (Arb.) 282 To solace your eares with pretie conceits after a sort of long scholasticall preceptes which may happen haue doubled them. 1665 HOOKEMicrogr. 9 They double all the Stuff .. that is, they crease it just through the middle .. placing the two edges, or selvages just upon one another. 1694 DRYDENLove Triumph. III. i, The page is doubled down. 1778 MAD. D’ARBLAYDiary 3 Aug., He doubled his fist at me. 1874 BLACKIESelf-Cult. 42 Bending his back, and doubling his chest. 1885 BIBLE (R. V.) Exod. xxvi. 9 Thou .. shalt double over the sixth curtain in the forefront of the tent. 1893 A. H. S. LANDORHairy Ainu 54 Crouched as she was, doubled up, with her head on her knees.
b. To double up (a person): to make to bend or stoop, as by a blow; hence fig. to finish up, cause to ‘collapse’. (slang or colloq.)
1814 Sporting Mag. XLIV. 278 Planting a blow on the side of Perrot, which doubled him up. 1883 J. PARKERTyne Ch. 108 Never saw a man so doubled up [in argument]. 1891 E. W. GOSSEGossip in Library xxi. 275 This master of science [pugilism], who doubled up an opponent as if he were plucking a flower.
c. intr. (for refl.) To become folded together or bent over; to fold, bend.
? 1650 Don Bellianis 164 With such terrible incounters that the knight .. doubled backward upon his horse. 1875 DARWINInsectiv. Plants vii. 163 After 10 hrs. 15 m. .. the blade quite doubled up. Mod. His knees doubled up under him. The leaf has been folded, and tends to double over.
d. Billiards. (a.) intr. Of a ball. To rebound. (b.) trans. To cause (a ball) to rebound: cf. DOUBLET 7.
1885 Billiards simplified (1889) 50 If you .. hit the red nearly full, so that it doubles down the table [etc.] Mod. You can double the ball into the middle pocket.
9. Naut. (trans.) To sail or pass round or to the other side of (a cape or point), so that the ship’s course is, as it were, doubled or bent upon itself.
1548 HALLChron., Hen. VIII. II b, If you wil bring your shippe into the bay of Hardines, you must double ye poynt of Gentilnes. 1585 T. WASHINGTON tr. Nicholay’s *Voy*. I. X. 12 B, Having doubled the cape, we passed along. 1665 Phil. Trans. I. 42 To go into the East Indies without doubling the Cape of Good Hope. 1867 FREEMANNorm. Conq. (1876) I. v. 295 The invaders doubled the Land’s End and ravaged Cornwall.
b. intr. To get round. To double upon (in naval warfare): to get round to the other side of (an enemy’s fleet), so as to inclose it between two fires.
1769 FALCONERDict. Marine (1789) A a ij b, The lee-line .. cannot so easily double upon the van .. of the enemy. 1856 Emerson Eng. Traits v. 91 Nelson’s feat of ‘doubling’, or stationing his ships one on the outer bow and another on the outer quarter of each of the enemy’s. 1867 SMYTHSailor’s Word-bk., Doubling upon .. a hostile fleet .. as Nelson did at the Nile. 1875 F. HALL in Lippincott’s Mag. XVI. 751/2 I doubled nimbly round a couple of corners, and paused again
10. intr. To turn sharply and suddenly in running, as a hunted hare; to turn back on one’s course; to pursue a winding or tortuous course.
1596 DRAYTONLegends ii. 382 To the Covert doth himselfe betake Doubling, and creepes from Brake againe to Brake. 1690 DRYDENAmphitryon IV. Wks. 1884 VIII. 75 See how he doubles, like a hunted hare. 1724 DE FOEMem. Cavalier (1840) 95 He found the river fetching a long reach, double short upon itself. 1828 D’ISRAELIChas. I, I. iv. 87 The nogociation doubled through all the bland windings of concession and conciliation. 1864 D. G. MITCHELLSev. Stor. 306 They suddenly turned to double upon their walk again.
11. fig. (intr.) To make evasive turns or shifts; to use duplicity, act deceitfully. ? Obs.
1530 PALSGR. 525 2, I double, I varye in tellying of my tale. . . Nay, and you double ones, I have done with you. 1578 HUNNISHyveful Hunney Gen. xii. 25 Why has thou dealt thus craftely And doubled so with mee>? 1624 Trag. Nero III. iii. in Bullen O. Pl. I. 54 Why with false Auguries have we bin deceiv’d? What, can Celestiall Godheads double too? 1649 Bounds Publ. Obed. (1650) 35 Who have been .. attent not to double with their God. 1820 SCOTTIvanhoe xxxv, If thy tongue doubles with me, I will have it torn from thy misbelieving jaws.