Kung fu or gongfu or gung fu (功夫) is a Chinese term often used by speakers of the English language to refer to Chinese martial arts. However in its original meaning, kung fu can refer to any skill. Gōngfu (功夫) is a compound of two words, combining 功 (gōng) meaning "achievement" or "merit", and 夫 (fū) which translates into "man", so that a literal rendering would be "human achievement." Its connotation is that of an accomplishment arrived at by great effort. The Chinese equivalent of "martial art" would be read as 武術 wushù.
　
Originally, to practice kung fu did not just mean to practice martial arts. Instead, it referred to the process of one's training - the strengthening of the body and the mind, the learning and the perfection of one's skills - rather than to what was being trained. It refers to excellence achieved through long practice in any endeavor. You can say that a person's kung fu is good in cooking, or that someone has kung fu in calligraphy; saying that a person possesses kung fu in an area implies skill in that area, which they have worked hard to develop. Someone with "bad kung fu" simply has not put enough time and effort into training, or seems to lack the motivation to do so.