费劲 (fèi jìn) describes a task that is strenuous and requires a lot of effort. It is very close to 费力 (fèi lì), but 费劲 often emphasizes the task's difficulty, while 费力 focuses on the effort you expend. Unlike 吃力 (chī lì), which is an adjective describing either a hard task or a person who is exhausted, 费劲 is a verb and can be split (e.g., 费了半天劲).