Harsusi

men - from; negative of an order; lest; than, than that; when, from the time that (HL 89)

PWS
*min - from (Arakelova 2001:39, Kogan 2015: 91–92)
Lipiński (1997:465) traces this preposition back to the PS root *mny ‘to count, to measure’, which is highly improbable (Arakelova 2001:41).