Full house sign: Abbr.

• Letters at a "Hamilton" show, probably
• Broadway abbreviation
• Sign of a big crowd
• Crowded house letters
• "All seats have been sold" sign
• Letters that signal a sold-out show
• Sign of theatrical success
• Sign at a B'way sellout
• Sign by a sold-out ticket booth

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