Especially when "kung fu" literally means something like "skill from effort" (as opposite of magic, praying, good genes etc). Kung = skillful work or hard training, Fu = time spent. It's mean pushing yourself to improve.
If we can trust Wikipedia:
> In its original meaning, kung fu can refer to any discipline or skill achieved through hard work and practice, not necessarily martial arts.
I can verify that as the correct translation and people in fact do use "kung fu" to mean a skill or an art you achieve through practice. In the part of China where my family is from, people do say "have no kung fu" to mean some practitioner is bad at what he or she does.
If we can trust Wikipedia:
> In its original meaning, kung fu can refer to any discipline or skill achieved through hard work and practice, not necessarily martial arts.
https://en.wikipedia.org/wiki/Kung_fu_(term)