A crony (rhymes with “pony”) is a buddy, a close confidant you spend time with and trust. Your boss might make everyone at work angry if she keeps only giving raises to her cronies.
While a crony is basically just a good pal or sidekick, the word sometimes has a negative connotation — that you and your crony are up to no good together. It also implies the idea of cronyism, or unfairly giving friends jobs or promotions they're not qualified for. Crony capitalism refers to doing that in an economic way, like people in government giving tax breaks to their friends. Crony is 17th century British student slang, from the Greek khronios, "long-lasting."