frighten his brother into fits
"Bill Starkey," continued John, "did not mean to frighten his brother into fits when he
dressed up like a ghost and ran after him in the moonlight; but he did;"

This quote is from Sewell's 'Black Beauty'. It talks about ignorance and mischievous acts. But the usage of "frighten his brother into fits" is a bit vague to me. Could you explain it in some detail? What is the relation between the meaning, which I guess is that he scared him, and the words used in that idiom or expression?

Question

frighten his brother into fits
"Bill Starkey," continued John, "did not mean to frighten his brother into fits when he
dressed up like a ghost and ran after him in the moonlight; but he did;"

This quote is from Sewell's 'Black Beauty'. It talks about ignorance and mischievous acts. But the usage of "frighten his brother into fits" is a bit vague to me. Could you explain it in some detail? What is the relation between the meaning, which I guess is that he scared him, and the words used in that idiom or expression?

Answer

"Frighten to fits" means to scare someone to the extent of producing uncontrollable emotions or body movements.

http://learnersdictionary.com/definition/fit
 4 fit /ˈfɪt/ noun
plural fits
Learner's definition of FIT
[count]
1 
: an uncontrolled expression of strong emotion

He threw a fit [=he became very angry and upset] when they accused him of cheating.
    She has/throws a fit [=tantrum] when she doesn't get what she wants.

2 
: a sudden occurrence of some activity, emotion, etc.

a fit of anger
    a fit of coughing = a coughing fit
    He apologized profusely in a fit of remorse.
    The joke sent the audience into fits of laughter. [=it made the people in the audience laugh a lot]

3 
: an abnormal state in which you become unconscious and your body moves in an uncontrolled and violent way

an epileptic fit [=(more commonly) seizure]