Wikipedia suggests two meanings for the physical action. The relevant one:
The ironic usage originates with the idea of suppressed mirth — biting one's tongue to prevent an outburst of laughter.
This matches with what I have always assumed was the answer: You are biting your tongue, cheek or lip to keep a straight face. The intent isn't so much to cause pain but to distract you from the joke or bluff. Another common behavior is to purse your lips when trying not to smile.
According to the Oxford English Dictionary, the use of strike in baseball was originally referred to as:
An act of striking at the ball, characterized as a fair or foul strike (see quot. 1874); three ‘foul strikes’ cause the batter to be put out.
The literal definition is (there is also the figurative one of having "a strike against you):
A ‘foul strike’, or any act or shortcoming on the batter's part which incurs the same penalty. Hence, a pitched ball recorded against the batter; esp. as one of three counts against the batter
It was first used in the 1800s:
1841 Picayune (New Orleans) 25 May 2/2 If ‘Edith’ wishes to see ‘a great strike’‥, let her walk down Water street‥and see the ‘bachelors’ make the ball fly.
So a strike in baseball comes from the attempt to strike the ball. It seems that it was used positively for a while--there are quotes referring to "great strikes". It looks like our current use of strike could be a shortening of foul strike--it only maintained its negative meaning. By the end of the 1800s, it still referred to the physical act of hitting something:
1896 R. G. Knowles & M. Morton Baseball 103 Strike.—When the batsman tries and fails to hit a ball delivered to him by the pitcher, or refuses to strike at a fair ball.
By the 1900s, however, it was a negative thing:
1912 C. Mathewson Pitching in Pinch 12 It put me in the hole with the count two balls and one strike.
This is the way we use it today.
Best Answer
In the past, to be left-handed was considered touched by the Devil. As Wikipedia notes:
So, if you were left-handed or sinister, you were associated with evil. In time, sinister itself meant evil and threatening. EtymOnline said that sinister attained this meaning in the early 15th century. The OED supports this, writing that the first uses of sinister to mean malicious were: