Learn English – Why are baseball statistics called “sabermetrics”

From the OED:

slang (orig. and chiefly U.S.). An undercover agent; a spy.

1942 BERREY & VAN DEN BARK Amer. Thes. Slang §458/16 ‘Spotter.’ (One who spys upon employees.).. Silent eye, spook, spotter. Ibid. §765/7 Rat, rubber heel, spook, spotter, a person employed to detect irregularities. 1954 People (Austral.) 3 Nov. 24/1 The spooks were senior constables who wore no uniform, worked in pairs and followed constables about the city and suburbs to see if they did their work properly. 1961 John o' London's 20 Apr. 434/1 The idea of making a living as a spy ‘spook’ in current Washington slang is repugnant to most of us. 1966 R. THOMAS Spy in Vodka (1967) vi. 50 I'd like him to get out of the spook business. 1979 L. PRYOR Viper i. 9 ‘My training was also in espionage at the CIA farm.’.. ‘A spook,’ I said in wonder.

This suggests the word would be mainly used in reference to American spies, but I think the 1954 reference is Australians talking about constables, so at least occasionally it has referred to any undercover person.

Not much information on why exactly the word was chosen, but it probably was an easy jump from "ghost that haunts people" to "mysterious secret agent who spies on people".

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.