Mr. Park's explanation is plausible.. but could you also have meant: 그냥 뻗었지! 뻗다 means to stretch out, relax, .. it can also mean to be knocked out, keeled over, including after drinking alcohol. :) I wonder what context you saw or heard this.

That is a wrong spelling of "그냥 빼앗았지" 그냥 means "just" 었지 is composed of 빼앗(다)+았+지 빼앗다 means "steal, rob, extort" 았 is past tense particle 지 is an ending particle. "그냥 빼앗았지" means "I just extorted it"