O.E. wenian "to accustom," from P.Gmc. *wanjanan (cf. O.N. venja, Du. vennen, O.H.G. giwennan, Ger. gewöhnen "to accustom"), from *wanaz "accustomed" (related to wont). The sense of weaning a child from the breast in O.E. was generally expressed by gewenian or awenian, which has a sense of "unaccustom" (cf. Ger. entwöhnen "to wean," lit. "to unaccustom"). The prefix subsequently wore off. Figurative extension to any pursuit or habit is from 1526.
My wife is a La Leche League leader, so the term "wean" is often heard around here. I thought the word history was interesting, with the prefix dropping over time...