口では大坂の城も建つ
(Kuchi de wa Oosaka no shiro mo tatsu;
“Even Osaka Castle can be built in words”)

Definition:

Mighty deeds are easy to talk about, but hard to actually accomplish. “Easier said than done.”

Breakdown:

We begin with the noun 口 (kuchi), “mouth,” followed by particle で (de), indicating “the means by which an action is performed,” then topic-marker は (wa), probably here serving a contrastive function. This is followed by noun 城 (shiro), “castle,” connected by associative particle の (no) to proper noun 大阪 (Oosaka), i.e. “Osaka.” This noun phrase is followed by emphatic particle も (mo), “also” or “even,” and finally the verb 建つ (tatsu), “to be built,” in sentence-final form.

Notes:

One variant replaces the castle with… a town? The part after は may be replaced with ～親船も造る (~oyafune mo tsukuru), and while the term 親船 looks like it should mean “mothership,” a quick search suggests that it’s mostly associated with a town named Oyafune. See also 口自慢の仕事下手 and 不言実行.

Example sentence:

「口では大坂の城も建つと言われても…計画を実行に移すためには、そもそも計画を立てないと何も進まないじゃないですか。まずは計画のブレインストーミングをさせてください」

(“Kuchi de wa Oosaka no shiro mo tatsu to iwarete mo… keikaku wo jikkou ni utsusu tame ni wa, somosomo keikaku wo tatenai to nani mo susumanai ja nai desu ka? Mazu wa keikaku no bureinsutoomingu wo sasete kudasai.”)

[“Even saying that words are just pie in the sky… if you don’t have a plan in the first place, you don’t have anything to implement, right? Please let me do some brainstorming first.”]