Now the spitball is outlawed.  The reason for ending the legal spitball was, in 1920, Carl Mays threw a spitball that hit Ray Chapman on the head, killing Chapman. 

Many pitchers used the spitball before 1920 and its unpredictable behavior made batters dread the pitch.  Even though it was outlawed, various pitchers have been charged with using it….but not by spitting on the ball.  They use more devious methods like moistening the hands and rubbing the ball.  Even ear wax has been used! 

The practice was first recorded in baseball around 1904 and the word, spitball, became a slang term in our language.  ‘To spitball’ means ‘to speculate’ in the stock market and elsewhere because

 

“no one ever knows the way a spitball will break, especially up or down’.  The way the stock market has been up and down of late, they must be ‘spitballing’ all over the place’! 

Childish spitballs have been in use longer than the baseball variety.  They were first recorded in 1846.  That stealth-like spitball behavior is still with us!

taken from the Henry Holt Encyclopedia of Word and Phrase Origins