Said after a very corny, very unfunny joke. Derived from the old Vaudeville comedy acts in which almost every one-liner was punctuated by a musical jingle. Usually played on the piano by a portly man with bright red cherub-like cheeks.
When you are overly high or drunk. This can be from several things ranging from nicotine to weed to lean to alcohol. To know if you are schmacked look yourself in a mirror and see if you can stand still. If this objective is failed then your SCHMACKED.