There are three voi words:
The A-infinitive form is voida ("to be able to") which in several inflected forms is voi. So it is an irregular verb just like that English counterpart.
Yes, as a noun it means butter.
Voi is also used as an amplifier in interjections, cf. "oh".
There are lot of mild expressions with voi. These are used when something annoys you, something has gone wrong or you wonder why something has been taken up as a topic in a discussion.
Voi voitettuja! : Vae victis!
Voi hyvänen aika (sentään)! :
- Voi pahus! : Oh damn!
Voi saamari! : Oh damn!
Voi ei! : Oh no!
- Voi taivas! : Oh (my) heavens! Good heavens! (N.B. the Finnish expression is not used for positive surprises)
Even milder are those with interjection voihan:
- Voihan kevätväsymys! : [lit.] Oh springtime lethargy!
- Voíhan nenä! : [lit.] Oh (my) nose!
- Voihan Venäjä! : [lit.] Oh Russia! (I am not sure but I think the word for Russia is only used for the rhyme's sake; Finns like alliterations)
- Voihan XX sentään! : For XX's sake! (here you can fill almost any noun, possibly with an adjective attribute; for instance Voihan kalsarienmyrkyttäjä sentään!, a translation of the Russian expression Владимир Отравитель трусов, Vladimir the pants poisoner)
Of course there are strong ones:
- Voi… [CENSORED]