It is the english conversion of the Latin phrase "post hoc ergo propter hoc" which simply means "after this, therefore because of this". It refers a illogical or false assumption of consequent occurence of the event due to or caused by happening of another event which took place prior to the second event .
[n] - the logical fallacy of believing that temporal succession implies a causal relation
Post hoc; ergo propter hoc. After this, therefore because of this. The logical fallacy that because one event follows another, the former must have caused the latter; or, the fallacy of arguing that something is the effect of a certain cause when there is no evidence of any connection.
post hoc ergo propter hoc noun the logical fallacy of believing that temporal succession implies a causal relation