yellow jacket noun Etymology:yellow (I) + jacket 1.oryellow hornet: any of various small yellow-marked social wasps of the family Vespidae that commonly nest in the ground — compare hornet 2.: any of several Australian eucalypts with yellowish bark: as a.: yellow box b.: a red gum (Eucalyptus rostrata) 3.[so called from the color of the capsules]slang: barbiturate 2