Everyone was surprised by this unexpected visitor.
Tips
history
From the Book of Changes (易经): 速 here means 'to invite' (an archaic meaning), not 'fast.' So 不速之客 literally means 'a guest who was not invited.' This idiom has been in use for over 2,000 years.
mistakes
Don't confuse 速 (invite, archaic) with 速 (fast, modern). The idiom does NOT mean 'a guest who is not fast.'