out of the blue

Từ điển WordNet v3.1 - WordNet Dictionary

I - adjective

not anticipated (Freq. 1)
- "unanticipated and disconcerting lines of development"- H.W.Glidden
- unforeseen circumstances
- a virtue unlooked-for in people so full of energy
- like a bolt out of the blue
• Syn:
unanticipated, unforeseen, unlooked-for
• Similar to:
unexpected

II - adverb

in a way that was not expected
- her brother showed up at the wedding out of the blue
• Syn:
unexpectedly
• Derived from adjective:
unexpected (for: unexpectedly)