Sign of being unable to sit down

• Bdwy. sellout sign
• "No seats" letters
• Problem for late arrivals, briefly
• Hit show sign
• Welcome sign for concert promoters
• Broadway hit letters
• Like a transient's qtrs., maybe
• Letters concerning available space
• Good sign for scalpers

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1941, initialism (acronym) for standing room only.