Related Clues:

• Viral inflammation, informally
• Cool, in the '50s
• Hardly old-fashioned, in an old-fashioned way
• Cadence shout
• Not at all square
• "In the old grooveroo"
• Like many a 21-Down
• Up on things, daddy-o
• Cool, as a daddy-o
• Cool, like a cat
• Hardly squaresville
• In the know, in old slang
• Lead-in for cat
• In the know, daddy-o
• Not square, once