'in-line' originates from two elements: Old English 'in' and Latin via Old French 'linea'. 'in' (Old English 'in') meant 'in, into', and 'line' (Latin 'linea') meant 'linen thread' or 'line'.

'in-line' developed from the combination of the preposition 'in' and the noun 'line' in Middle English (often written as the phrase 'in line'); over time the phrase was sometimes hyphenated as 'in-line' and later also written as the single word 'inline' in technical contexts.

Initially it referred mainly to literal placement 'in a line' (physical alignment). Over time it broadened to figurative senses such as 'in agreement' and specialised technical senses like 'inserted within the main text or code' (e.g., inline image or inline function).