Chum/tʃʌm/ Definition 1 of 4Pro Tip 1/3Informal Use 'Chum' is casual and best used among friends, not in professional settings. I'm catching up with my chum after work. Pro Tip 2/3British Origin 'Chum' is more common in British English and can seem old-fashioned or endearing in American English. My chum at university recommended this book. Pro Tip 3/3Age Neutral 'Chum' can denote friendship in any age group, from children to the elderly. Our grandpas are old chums who love chess.