a good egg
Someone who is known to be trustworthy and kind. Joey's a good egg. He always makes time to help others in need.
a good eggOLD-FASHIONED
If you describe someone as a good egg, you mean that they are a kind and reliable person. Her father's a bit bad-tempered, but basically a good egg.
good egg, a
An agreeable, trustworthy person. This slangy expression has outlived bad egg, which it actually implied in the sixteenth century. “Neither good egge nor good bird,” went the saying, meaning the young (egg) would not turn into praiseworthy adults (bird). In the nineteenth century this continued to be spelled out: “A bad egg [is] a fellow who has not proved to be as good as his promise” (The Athenaeum, 1864). The favorable aspect of good egg dates from the early twentieth century. Rudyard Kipling used it in Traffics and Discoveries (1904): “‘Good egg!’ quoth Moorshed.”
See also: good