Вилами на воде писано (vilami na vode pisano).
Literally: written with pitchfork on the water.
Meaning: it is unknown how something is going to be in the future.
English equivalents: up in the air; not written in stone.
1) Although in contemporary Russian ‘vily‘ (pl.) means pitchfork it has also another meaning: South Slavic female spirits who live in ponds (sing.: vila; cf. mermaids). They can tell fortune writing what is going to be on the water.
2) Until now in some Russian dialects ‘vily‘ also means circles on the water. While fortune telling stones were thrown in the water. Prediction was based on the form, size of the resulting circles as well as the way they crossed.
3) There was a pagan ritual: a defensive spell against vodyanoy (male water spirit) when Russian peasants drew a cross on the water using a knife or a scythe. The latter were symbols of the god called Perun. Perun was the god of thunder and lightning (cf. Thor, Zeus, Indra, etc.). He was the highest god of the Slavic pantheon.
4) According to another version this saying is a loan translation of a Greek phrase γράφω στο νερό, i.e. to write on the water which has a figurative meaning – to do something useless or meaningless.