grimaco

One root, whatever it looks like. Grimaco invites a decomposition and refuses every one of them. There is no grim- root and no -ac- suffix in official Esperanto; the unofficial -aĉ- with a hat means shoddy or nasty, which is close enough in feel to make the guess tempting and is still not what is happening here. The word arrived whole from French.

Which is a shame, because the real history is better. French grimace goes back to Frankish grima, a mask. A grimace is a face you put on. Old English had the identical word for a mask or the face-guard of a helmet, and it survives, worn thin, in the adjective grim, which once meant fierce rather than merely bleak.

The fence with mieno. Mieno at rank 692 is the expression on a face, neutral: you can have a cheerful mieno or a serious one. A grimaco is distorted, and usually deliberately or involuntarily so. Pain, disgust, a sour drink, a child mocking a teacher: all grimaco. Grimaci is the verb, so li grimacis is he pulled a face.

A small vocabulary of the face. Vizaĝo at 172 is the face itself, rideti at 345 is to smile, sulkigi la brovojn is to frown, and this word covers the rest of the range. It is a surprisingly useful thing to be able to name in a language that gets spoken at noisy congresses.