1. To join in attaining a common aim

Derived forms: mucked in

• underactive
• underage
• underarm
• underbelly
• underbid
• underbodice
• underbody
• underboss
• underbred
• underbrush
• undercarriage
• undercharge
• underclass
• underclassman
• underclothed
• underclothes
• underclothing
• undercoat
• undercoated
• undercook
• undercover
• undercover agent
• undercover operation
• undercover work
• undercroft