a term that Japanese people say when using a decent amount of physical power, especially when lifting up something heavy or standing up from a chair
Also よっこいしょ. For many Japanese people (especially old people), it’s almost like a habit to say this term when using physical power, and they often say this term unconsciously. See the post about よいしょする for its slang usage.
Hmm, this box looks heavy. Yoisho! (lifting it up)