Idiom Definition:
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"off the top of your head"  
    to say something without thinking about the subject too much, relying on memory only; extemporaneous    
            
Synonyms: 
ad-lib, extemporaneous, extempory, fake, free, impromptu, improvise, improvisatory, made-up, makeshift, offhand, off-the-cuff, on-the-spot, snap, spontaneous, spur, unplanned, unpremeditated, unprepared, unrehearsed, unstudied, winging
 
         
 
