This chess player is no slouch - even the champion lost to him.
Tips
usage
Almost always negated. The standard frame is 不是等闲之辈 / 非等闲之辈 - 'is no ordinary person.' Saying someone IS a 等闲之辈 is grammatical but rare and dismissive.
history
Recorded in Luo Guanzhong's 《三国演义》: 魏有邓艾，足智多谋，非等闲之辈 - 'Wei has Deng Ai, resourceful and shrewd; he's no ordinary person.'
