fast asleep – English Grammar Profiler
‘fast’ means ‘completely’ when used with the adjective ‘asleep’
This phrase is C2 in the English Vocabulary Profile.
For example:
Look, Charlie, she‘s fast asleep.
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A NOW corpus collocates search for ‘fast asleep’
1 WAS 1032 2 WERE 498 3 WHEN 376 4 WHILE 181 5 BED 114 6 FELL 59 7 BABY 41 8 ROOM 40 9 MOTHER 34
10 WOKE 31