Nosh is a Yiddish word that means 'snack'. It can be used as a noun ("Would you like a little nosh?") or a verb ("They noshed on pretzels while watching the movie""). It is sometimes also used as a synonym for 'eat' or 'meal'. When used to mean 'snack' it is often qualified as 'a little nosh', although strictly speaking this is redundant.
Nosh is generally considered slangish (although it's worth noting that my spell checker recognizes it). It comes from the Yiddish nashn or nash (נאַשן) which means to eat sweets or to nibble. Nash comes to Yiddish from the High German naschen. Nosh is conjugated just as you would any English word; noshed, noshing, noshes. It is pronounced just as it looks like it should be, /nɒʃ/ for those of you who can read IPA.
Nosh is also an Algonquin name given to boys. Its literal meaning is 'father'. (While it may seem odd to name a baby 'father', it's not uncommon. In Aramaic the name Avina means father, in Arabic it's Abba, in Hebrew it's Abbot, Aviel, or Avi, in Navajo it's Shizhe'e, and in English we have Abraham, which is often translated as 'father of many'.)