The term spew is used to refer to an overly aggressive play, in which the overplayed hand is beaten by a better hand. Apart from that, the term can also refer to loose call downs, i.e. situations in which a player doesn't hesitate to call his opponent's bets although he only has low showdown equity.
A player is referred to as spewy if he plays a noticeably aggressive game and metaphorically spews his chip stack over the table, giving it to other players with better hands. It is usually loose players who are spewy; Nits on the other hand will less frequently insist on making decisions that aren't likely to be successful, since such players tend to think about their moves more thoroughly.