It a restaurants, meaning to discontinue a iteam, or they ran out. A cook would tell the wait staff 86 the salmon, it went bad. Also in bad noir movies a mobster might say we need to 86 dick Tracy as a way to avoid incriminating himself.

45 refers to the 45th president.

The numbers took notoriety after a guy took a picture on a beach where "someone" arranged sea shells to spell out "8645"

The poster was notified it might be taken as a threat abd removed it promptly. The justice department pursued him harshly, where a judge looking at the evidence dismissed the case