Diodorus Siculus
"Physically the Celts are terrifying in appearance with deep-sounding and very harsh voices.  In conversation they use few words and speak in riddles, for the most part hinting at things and leaving a great deal to be understood.  They frequently exaggerate with the aim of extolling themselves and diminishing the status of others.  They are boasters and threateners, and given to bombastic self-dramatization, and yet they are quick of mind and with good natural ability for learning." (Book 5)