at once

1.  At the same time, as in We can't all fit into the boat at once. [First half of 1200s] Also see at one time, def. 1.
2.  Immediately, as in Mother told the children to come inside at once. [First half of 1500s]

The adverb has 2 meanings:

Meaning #1: without delay; with no time intervening
  Synonyms: immediately, instantly, straightaway, straight off, directly, now, right away, forthwith, in real time

Meaning #2: simultaneously
  Synonyms: at a time, at one time