Borrowed from Russian га́йка (gájka), from Proto-Slavic *gajьka. Further origin is uncertain: * Either related to Bulgarian жир (žir, “fat, grease”), гоя́ (gojá, “to lush”) with initial meaning something that fastens, fixates (compare Ukrainian га́їти (hájity, “to hinder one's motion”), Czech hájit (“to protect”)); * or from the root of Bulgarian га́щи (gášti, “pants”), originally meaning something that fits onto. See Russian га́йка (gájka) for further discussion and cognates.