Korean beggars used to beg with dried, hollowed out goard halves, but during the Korean War and for somethime afterwards, beggers used empty tin cans (깡통) that were apparently attached (차다) to themselves in some way. Therefore, the expression 깡통을 차다 triggers images of a time during and after the Korean war rather than a time before the war.
Today, 깡통(을) 차다 is used idiomatically to mean the following:
- to be reduced to begging
- to go bankrupt
- to go bust
- to go to pot
By the way, if 깡통 means "empty can," why do Koreans call a "can opener" a 깡통 따개? Wouldn't an "empty can" already be opened? Likewise, 깡통 맥주 (can beer) does not make sense, either, since it literally means, "empty can beer."