jumi

• IPA^(key): /ˈjumi/, [ˈjumi]
• Rhymes: -umi
• Hyphenation^(key): ju‧mi

From the same root as jumissa. The sense "muscle stiffness" is probably a relatively recent backformation from jumissa.

jumi

1. (informal) (muscle) stiffness, e.g. in the neck
2. (dialectal) slow, sluggish, lethargic person

• jumiin
• jumissa
• jumittaa
• jumittua
• jumiuttaa

• “2. jumi”, in Kielitoimiston sanakirja [Dictionary of Contemporary Finnish]‎^[1] (in Finnish) (online dictionary, continuously updated), Kotimaisten kielten keskuksen verkkojulkaisuja 35, Helsinki: Kotimaisten kielten tutkimuskeskus (Institute for the Languages of Finland), 2004–, retrieved 2023-09-14

Possibly from how the animal appears to be stuck, in which case of the same origin as jumi (etymology 1).

jumi

1. wood borer, woodworm (any of several insects and their larvae in the family Anobiidae)

• “1. jumi”, in Kielitoimiston sanakirja [Dictionary of Contemporary Finnish]‎^[2] (in Finnish) (online dictionary, continuously updated), Kotimaisten kielten keskuksen verkkojulkaisuja 35, Helsinki: Kotimaisten kielten tutkimuskeskus (Institute for the Languages of Finland), 2004–, retrieved 2023-09-14

Probably akin to jumala.

jumi

1. (Finnic mythology) A kind of supernatural creature.