From Latin 'ad-' + 'simul' (together), PIE *sem- (one) — the same root that produced 'similar,' 'simultaneous,' and 'simple.'
To come together or bring together into a group; to fit together the component parts of something.
From Old French "assembler" meaning "to gather together, bring into one place," from Vulgar Latin *assimulāre, a reshaping of Latin "assimulāre" (to make similar, to bring together), itself composed of the prefix "ad-" (to, toward) and "simul" (together, at the same time). Latin "simul" traces back to Proto-Indo-European *sem- (one, as one, together), a root of extraordinary productivity across the Indo-European family. The PIE root
In computing, 'assembly language' (1950s) is the lowest-level human-readable programming language — it 'assembles' machine instructions. The name reflects the physical process of early computing, where programmers literally assembled programs from individual instruction codes.