'same-direction' is a compound of 'same' and 'direction'. 'same' originates from Old English 'same' (sēam), meaning 'identical' or 'equal', and 'direction' originates from Latin 'directio' via Old French and Middle English, where 'directio' referred to 'a leading or guiding'.

'same' remained relatively stable from Old English into Modern English; 'direction' came into English from Latin 'directio' through Old French 'direccion' and Middle English 'direccioun', and the two elements have been combined in modern English as a compound ('same-direction') to describe matching orientation.

Initially, 'same' meant 'identical' and 'direction' meant 'the act of guiding or pointing'; over time the compound came to be used specifically to indicate shared orientation or movement — 'pointing or moving in the same way'.