功夫(gōng fu) - skill acquired through effort, kung fu, time and effort in Mandarin

功夫 (gōng fu) is the effort you put into something over time, and the skill that effort produces. 下功夫 is to put the work in, 花功夫 is to spend time on something, and 有功夫 is to be good at something because you did. The martial-arts sense that English borrowed is only one branch of this.

That English borrowing is the source of the main trap. "Kung fu" in English means fighting; 功夫 in Chinese means mastery of anything, so a calligrapher, a cook and a translator can all be said to 有功夫. If you specifically mean the martial art, 武术 or 中国功夫 removes the ambiguity.

There is also a time sense to watch, spelled the same and often written 工夫 instead. 三天的功夫 is a span of three days, and 有功夫吗 asks whether you are free. Context sorts it out, but do not be surprised when the word turns up nowhere near a training hall.