Initials on a packed house

• Broadway letters for a hit show
• Like a hit show
• Stage sellout sign
• Box-office sign, briefly
• Sellout indicator
• Sign at a hit
• Capacity descriptor, briefly
• Fleabag hotel, for short
• Sign of a sell-out

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