一寸の虫にも五分の魂
(Issun no mushi ni mo gobu no tamashii;
“Even in a one-sun bug, five parts of a soul.”)

Definition:

Even a 3cm bug has a half-share of spirit. Even a seemingly small, weak person will have a certain underlying stubbornness or strength of spirit. Don’t look down on the small or weak. “Even a worm will turn.” As with other sayings, the specified length is not to be taken literally; instead it stands as an example of something very small… despite the fact, as any entomologist can tell you, that 3cm is pretty large for many “bugs.”

Breakdown:

一寸 (issun) is one sun (pronounced somewhat close to “soon”), or about 3cm in modern terms. This is joined by the associative particle の (no) to the next noun, 虫 (mushi), a broad category of animal roughly equivalent to the common American English use of “bug.” (Mushi is a category that historically includes not only insects, spiders, and other arthropods, but also other creepy-crawlies such as snakes. The modern usage is probably more tightly restricted to insects and insect-like creatures.)

The whole noun phrase is pointed to by the multi-use particle に (ni), here serving a sort of attributive or possessive function, combined with も (mo), “also” or “even.” And this particle cluster joins the first noun phrase with another: 分 (bu) means a “part,” but as I’ve said before [https://landofnu.com/2014/09/24/nine-and-nine-of-ten-and-ten/], it can also refer to one tenth of a whole. 五分 (gobu) is therefore 5/10, or 1/2. And again our measure is connected by の to another noun, in this case 魂 (tamashii), “soul” or “spirit.”

Notes:

The も can be replaced by さえ (sae), which indicates some minimum limit and can often be translated as “even.” The whole saying can also be invoked by the truncated form 五分の魂 (gobu no tamashii).

A similar saying with the same meaning notes that 蛞蝓にも角 (namekuji ni mo tsuno); “even a slug has horns.”

Example sentence:

「相手はただの高校生だとは言え、なめるな。一寸の虫にも五分の魂があるというんだから」

(“Aite wa tada no koukousei da to wa ie, nameru na. Issun no mushi ni mo gobu no tamashii ga aru to iu nda kara.”)

[“Even if your opponent is just a high-school student, don’t underestimate them. As they say, even a bug has a half-share of spirit.”]