From Wiktionary, the free dictionary
siitä vain
- go ahead, go for it, feel free, be my guest (expression of acceptance for someone to do something)
Tiedät jo puhelinnumeroni. Jos tulee jotain asiaa, niin siitä vaan.- You already know my phone number. If something comes up, feel free (to call).
- come on, go ahead, be a devil (expression of encouragement to a person that is hesitating, particularly to do a specific action right away)
Siitä vaan, ei se satu.- Come on, it won't hurt.
- come on, let him/her/them/it come (expression of defiance)
Siitä vaan! Tästä he eivät pääse.- Let them come! They won't get through here.
siitä vain
- (idiomatic) on the spot, ad hoc
