What is the difference between 脅す, 脅かす, 脅迫する, 脅迫, 脅し

Question

What is the difference in meaning between 脅す, 脅かす, 脅迫する, 脅迫, 脅し?

How would each one be used in real life?

Answer 1

I will explain their differences in three stages, and I'll show their usage at the end.

(1) Origin
At first they are divided into two groups:
Group 1: 脅す, 脅かす, 脅し
Group 2: 脅迫する, 脅迫
Words in the first group belong to 大和言葉{やまとことば} or 和語{わご}, which is explained here.
The essence of the explanation is "大和言葉{やまとことば} literally "Japanese words" or wago are native Japanese words, meaning those words in Japanese that have been inherited from Old Japanese, rather than being borrowed at some stage."

Words in the second group belong to Sino-Japanese_vocabulary or 漢語｛かんご｝, which is explained here.

The essence of the explanation is "Sino-Japanese vocabulary or kango refers to that portion of the Japanese vocabulary that originated in Chinese or has been created from elements borrowed from Chinese."

Generally, Yamato_kotoba or wago has a nuance of being emotional as a feature, so it is suitable for use in conversations or everyday expressions. On the other hand, kango has a logical nuance so it is suitable for use in official texts/documents, and it is easy to construct compound words together with other kango as its general linguistic nature.

Compound words using 脅迫:

• 脅迫罪{きょうはくざい}　criminal intimidation
• 脅迫状{きょうはくじょう}　a threatening letter
• 脅迫電話{きょうはくでんわ}　a threatening （telephone） call

(2) Part of class
脅す and 脅かす are verbs and 脅し is a noun having the same etymology meaning "threat".
脅迫する is a verb and 脅迫 is a noun having the same etymology meaning "threat".

(３) The difference between 脅す and 脅かす
The explanation here is cited from here. For details, please refer to the original text.

• 脅かす has two pronunciations as 脅｛おど｝かす and 脅{おびや}かす, and they are two different words.
• 脅{おど}かす and脅{おびや}かす has the same meaning to frighten others with some means.
• 脅{おど}かす is also used in a sentence like "突然{とつぜん}わっと言｛い｝って脅｛おど｝かす", meaning to surprise others.
• 脅{おびや}かすalso has meaning to make something or the other party in an unstable state by being used in a sentence like "首位{しゅい}の座｛ざ｝を脅｛おびや｝かす to make the leading position in an unstable state" or "平和｛へいわ｝を脅｛おびや｝かす to make the peace unstable or to threaten the peace".
• (natural) ピストルで人質｛ひとじち｝を脅｛おど｝かす to threaten the hostage with a pistol
• (natural) ピストルで人質を脅｛おど｝す
• (?) ピストルで人質を脅｛おびや｝す
• (natural) 大声{おおごえ}で子供｛こども｝を脅｛おど｝かす to frighten children with a loud voice"
• (natural) 大声で子供を脅｛おど｝す
• (unnatural) 大声で子供を脅｛おびや｝かす
• (natural) 社長｛しゃちょう｝を脅｛おど｝かして金｛かね｝を取る to threaten the president and deprive him of his money
• (natural) 社長を脅｛おど｝して金を取る
• (unnatural) 社長を脅｛おびや｝かして金を取る
• (unnatural) 核兵器{かくへいき}で隣国｛りんごく｝を脅｛おど｝かす
• (unnatural) 核兵器で隣国を脅｛おど｝す
• (natural) 核兵器で隣国を脅｛おびや｝かす to threaten neighboring countries with nuclear weapons

EDIT

I'll show the difference between 脅{おど}す, 脅｛おど｝かす and 脅｛おびや｝かす in more detail.

脅｛おど｝す means to give threat with concrete things like a knife or a gun that certainly gives threat effectively to the other party at a glance, to give threat with exposing the other party's critical weaknesses or shames, or to give threat with threatening what he/she doesn't want to lose, and the characteristics of 脅｛おど｝す are direct and static.

脅{おど}かす is akin to 脅｛おど｝す, but the means that is used in 脅{おど}かす is insufficient to give threat to the other party, so additional threatening words or acts are needed, and its characteristics are direct and noisy.

On the other hand, 脅｛おびや｝かす has somewhat different nuance from 脅{おど}す or 脅｛おど｝かす, and the target which 脅｛おびや｝かす threatens is different from that 脅{おど}す or 脅｛おど｝かす does.
脅｛おびや｝かす has a nuance or an image in which an enormous threat or fear not having an obvious appearance gradually invades the target. And, the target which 脅｛おびや｝かす threatens is not a person but abstract things such as a society, a nation, a status, peace, happiness and the like.

Answer 2

脅迫【きょうはく】 is a kango (Sino-Japanese word) whereas 脅す【おどす】/脅かす【おどかす】 is a wago (yamato-kotoba, native Japanese word). As always, the kango version looks stiff and technical. Specifically in this case, 脅迫 is an official name of a criminal offense ("intimidation"). So using 脅迫 for an argument between children may be over the top.

脅す (transitive verb) only means "to threat" or "to intimidate". 脅し is a nominalized version of 脅す (masu-stem as a noun). 脅かす (transitive verb) works as a bit colloquial equivalent of 脅す, and it can also mean "to frighten" or "to startle".