dikfingro

Thick-finger. That is a thumb. Dika is thick or fat, at rank 789. Fingro is a finger, at rank 663. The thumb is the fat one, and Esperanto says exactly that with no further ceremony. English thumb and German Daumen both go back to a root meaning the swollen or thick one, so the naming instinct is the same everywhere - the difference is that here you can see it.

The sibling is piedfingro at rank 2,605, foot-finger, a toe. Between them the two compounds make the point that page already made at length: if you owned piedo and fingro, you owned toe without meeting it. If you own dika and fingro, you own thumb. English charges you a separate opaque word for each; French charges you orteil and pouce. Esperanto charges nothing.

The whole hand, since the set is regular:

Five names, five compounds, and every element is a word you already have: montri to show, meza middle, ringo a ring, and the diminutive -et-. Learn the pattern once and the hand is done. The big toe, incidentally, is dika piedfingro or grandfingro, which is the fat finger of the foot - a compound of a compound, and still transparent.

The unmarked reading. Fingro on its own means a finger of the hand. Manfingro exists for the rare sentence that needs to be explicit about it, usually one that also mentions toes, but nobody says it casually. And the nail on the end of any of them is an ungo, rank 2,218, which covers fingernails, toenails and claws with one root.

The gesture. Levi la dikfingron is to give a thumbs-up and reads exactly as it does in English across most of the Esperanto-speaking world. Sub la dikfingro - under the thumb - also translates, though it is a borrowing of the English idiom rather than anything native. Mezuri per la dikfingro, measuring by thumb, is the rule of thumb.