Meaning

adverb (used to introduce a logical conclusion) from that fact or reason or as a result

adverb as a consequence

originFrom Middle English therfore, therfor, tharfore, thorfore; synchronically a univerbation of there (pronominal adverb) + for, literally “for that (reason)”. The spelling has been changed due to a reanalysis as there + fore (literally “forward from that; thence”). See also therefor, ultimately the same formation. Compare Saterland Frisian deerfoar, Dutch daarvoor, German dafür, Danish and Norwegian…

Etymology from Wiktionary, CC BY-SA 3.0