Vaiva - rainbow in Lithuanian.
Iris - rainbow in Greek
Solongo - rainbow in Mongolian.
For future reference, click the "advanced search" tab. There you will find a whole bunch of options, such as origin, length community impression and so forth.
When you find it, click the "Meanings and Description" button, type in "rainbow" in the meaning section, press enter and you're good to go :)
"When you're curious, you find lots of interesting things to do..." -
Albert Einstein