Old English 'withinnan' — 'with' (toward) + 'innan' (inside), the spatial counterpart of 'without.'
Inside the boundaries or limits of; not exceeding a specified amount of time or distance; in the inner part of.
From Old English 'wiþinnan,' a compound of 'wiþ' (against, toward, facing) and 'innan' (on the inside, from inside, within), from 'inn' (in, inside), from Proto-Germanic *in, from PIE *en (in, within). The preposition 'wiþ' in Old English had a much wider range than Modern English 'with' — it could mean against, toward, in relation to, or in exchange for, reflecting PIE *wi- (apart, in two). The 'wiþ' of 'within' and 'without' is directional and relational, not the comitative sense of accompaniment we now use 'with' for. The spatial counterpart 'wiþūtan' (without, outside) has undergone a dramatic semantic shift — 'without' now primarily means 'lacking' rather than 'outside' — while
The pair 'within/without' was originally a perfect spatial antonym in Old English — inside versus outside. But when 'without' drifted to mean 'lacking,' the symmetry broke. English never fully repaired it: 'within' still means 'inside,' but 'without' no longer means 'outside' in standard English. The ghost of the original pair survives in literary phrases like 'enemies within and without.'