An Etymological Dictionary of Astronomy and Astrophysics

1. Any of various protective head coverings worn by soldiers, policemen, firemen, etc.
   

2. Something resembling a helmet in form or position. → helmet streamer.

Etymology (EN): From M.Fr. helmet, diminutive of helme “helmet,” from Frank. *helm (cf. O.H.G. helm “helmet”);
PIE base *kel- “to cover, to hide;” cf. Av. sar- “shelter;” Laki šârd “hidden, hiddenly,” šârden “to hide;” Kurd. šâr-, šârdinawa “to hide;” Skt. śárman- “cover, protection, refuge;” L. celare “to conceal;” Goth. huljan “to cover, conceal;” O.H.G. helan “to hide.”

Etymology (PE): Xud “helmet,” from O.Pers. xaudā- “hat, cap,” tigra-xauda- “wearing the pointed cap” (as is shown in the sculpture of Skunkha the Scythian at Behistan); Av. xaoδa- “hat, cap, helmet;” Ossetic xodä; Arm. (borrowed) xoir “headband.”