The word 'therefor' is a direct descendant from Old English 'þǣrfor', a compound of 'þǣr' (there) and 'for' (for, on account of). It originally functioned as a straightforward adverbial phrase meaning "for that (thing), for it." Over time, this compound became lexicalized as a single word. Its meaning has remained remarkably stable, retaining its core sense of indicating purpose or reason in relation to a previously mentioned thing. It is now primarily used in formal, legal, or archaic contexts, having been largely supplanted in everyday speech by the two-word phrase 'for it' or the more common 'therefore'.

This word is a fused compound of the Old English demonstrative pronoun/adverb 'þǣr' (there) and the preposition 'for' (for, because of). It functions as a single lexical unit.