for (prep.) Look up for at Dictionary.com
Old English for "for, before, on account of," from Proto-Germanic *fura (cognates: Old Saxon furi "before," Old Frisian for, Middle Dutch vore, Dutch voor "for, before;" German für "for;" Danish for "for," før "before;" Gothic faur "for," faura "before"); see fore (adv.).

Use of for and fore gradually was differentiated in Middle English. Its use alone as a conjunction (not found before 12c.) probably is a shortening of common Old English phrases such as for þon þy "therefore," literally "for the (reason) that."