Origins
The word 'double' entered English in the early thirteenth century from Old French 'doble' (modern French 'double'), descended from Latin 'duplus,' meaning 'twofold.' The Latin compound joins 'duo' (two) with the suffix '-plus' (fold), from PIE *-plo- (fold) — making 'double' literally 'two-fold,' a transparent description of its meaning.
The PIE root *dwóh₁ (two) is one of the most securely reconstructed words in the proto-language, with reflexes in every branch. Germanic: 'two' (English), 'zwei' (German), 'twee' (Dutch), 'tvá' (Old Norse). Latin: 'duo' (two). Greek: 'dýo' (δύο). Sanskrit: 'dvā.' Old Irish: 'dau.' Lithuanian: 'du.' The word is so stable across languages and millennia that it served as one of the foundational correspondences in the development of comparative linguistics.
Through Latin 'duo' and its derivatives, the PIE root generated a large English word family. 'Dual' (consisting of two), 'duo' (a pair), 'duel' (a fight between two), 'duet' (music for two), 'duplex' (twofold, a two-unit dwelling), 'duplicate' (to make a second copy), and 'duplicity' (double-dealing, deceit — literally two-facedness). The word 'doubt' is also related: Latin 'dubitāre' (to waver, to be uncertain) derives from 'dubius' (wavering), from 'duo' — to doubt is to be of two minds, pulled in two directions.
Proto-Indo-European Roots
Through the Germanic branch, *dwóh₁ produced 'two,' 'twin' (one of two born together), 'twelve' (two remaining after ten), 'twenty' (two tens), 'twice,' 'twain' (an archaic form of 'two,' as in 'never the twain shall meet'), 'twilight' (the 'between' light — the uncertain period between day and night), 'between' (by two), and 'twill' (a fabric woven with a double thread).
The word 'double' itself has developed rich figurative senses. A 'body double' or 'stunt double' in film stands in for the principal actor. A 'double agent' serves two masters. 'Double-dealing' is deceitful behavior. 'Double-edged' describes something with benefits and risks. 'Double entendre' (from French, 'double understanding') is a phrase with two meanings, one of them risqué. 'To see double' is a symptom of intoxication or visual disorder. 'Double or nothing' is a gambling term. Each figurative extension exploits a different aspect of twoness: substitution, duplicity, ambiguity, or multiplication.
In music, 'double' has specific technical meanings. A 'double bar' marks the end of a section. A 'double bass' plays an octave below the cello (doubled down in pitch). A 'double flat' lowers a note by two semitones. In card games, 'to double' (especially in bridge) is to increase the stakes. In baseball, a 'double' is a hit that allows the batter to reach second base.
Legacy
The concept of doubling — making two where there was one — has been philosophically and psychologically significant since antiquity. The 'doppelgänger' (German for 'double-goer') is a supernatural duplicate of a living person, an uncanny figure in folklore and literature. Dostoevsky's 'The Double' (1846) and Stevenson's 'Strange Case of Dr Jekyll and Mr Hyde' (1886) explore the psychological horror of the doubled self. The word 'double' thus carries not only the neutral arithmetic of multiplication but also the deeper anxieties of duplication, division, and deceit.