Real skill and competence are apparent without needing to declare themselves, so humility is the mark of a true master.
(Kuchi jiman no shigoto beta; “Boastful mouth, bad at work”)
If you’re always working your mouth, you’re never working your hands. Describes someone who can talk a good game, but who is completely useless when it comes to doing real work. Someone who can talk the talk but can’t walk the walk. Someone so focused on talking about something that they never get around to learning how to actually do it. For example, a shameless self-promoter who gets and stays rich thanks to inheritance, theft, fraud, and an army of unethical accountants, yet claims to somehow be good at business or negotiation despite not knowing the first thing about actual negotiation or having any actual sense for how the economy works.
We begin with the noun 口 (kuchi), “mouth,” coupled with 自慢 (jiman), “pride” or “boasting.” The latter half of the phrase comprises 仕事 (shigoto), “work,” and 下手 (heta, with the h voiced to make beta as often happens in compound nouns), “unskilled.” These two noun phrases are connected by the associative particle の (no).
This saying apparently comes from an early Edo-period kanazoushi book (an often-didactic genre) titled 為愚痴物語 (Iguchi monogatari).
A related saying is 口叩きの手足らず (kuchi tataki no te tarazu, roughly “Someone who flaps their mouth lacks in their hands.”
(“Jinrui wa kuchi jiman no shigoto beta to iu no ga fuhenteki da shi, onore no gyouseki wo hometataeru seijiya ni wa zettai touhyou shinai koto ni kimeta.”)
[“’More talk, less skill‘ is a universal human trait, so I’ve decided never to vote for a politician who goes around praising their own achievements.”]