French contraction of "Vois-la" ("See there"), used in English to call attention to something, or to suggest an appearance as if by magic. The pronunciation is by French rules (as if spelled "vwah-lah" to English speakers) and so it is frequently misspelled in English -- as "viola" by people who've seen it written without hearing it, and "wallah" or something similar by those who've heard it without seeing it written.