"Wood for sheep" is a trade offer in the famous board game "The Settlers of Catan", where one player wants to trade his resource card showing a sheep against some other player's resource card showing a piece of lumber.
Obviously, this can jokingly hint to "wood" / "woody", ie. an erect male penis, for some "sheep", referring to some furry female body-part, ie. vagina.
So "wood for sheep" is a naughty way of insinuating coitus.
It has recently been shouted out on the famous TV show "The Big Bang Theory" when the cast played "The Settlers of Catan".