An Etymological Dictionary of Astronomy and Astrophysics

1. A declaration of an intention or determination to inflict punishment, injury, etc., in retaliation for, or conditionally upon, some action or course; menace.
   

2. An indication or warning of probable trouble (Dictionary.com).

Etymology (EN): M.E. threte, O.E. threat “pressure, oppression;” cognate with O.N. thraut “hardship, bitter end,” Du. verdreiten, Ger. verdrießen “to vex,” L. trudere “to press, thrust.”

Etymology (PE): Harš, from Kurd. haraša “threat,” haraša kirdan “threaten,” related to Mid/Mod.Pers. rašk “envy, jealousy;” Lori, Laki erešt “assault, attack;” Tabari ârâšt “curse, anathema;”
Av. arš- “to be envious;” Skt. īrs- “to be envious, envy;” Arm. her “anger, quarrel;” O.E. eorsian “to be malicious;” Proto-Ir. Harš- “to be envious.”