From Wiktionary, the free dictionary
в (v) + прок (prok, “benefit, use”)
впрок • (vprok)
- for future use, in reserve, in store
- to one's benefit
- э́то тебе́ не пойдёт впрок ― éto tebé ne pojdjót vprok ― it won't do you any good
впрок • (vprok)
- it is to someone's benefit, it profits
- ему́ впрок ― jemú vprok ― it's going right for him
