The definition of grievous is something very serious, grave or severe.(adjective)
A serious mistake that could cost someone his life is an example of a mistake that would be described as grievous.
See grievous in Webster's New World College Dictionary
Origin: ME grevous < OFr < grever: see grieve
See grievous in American Heritage Dictionary 4
Origin: Anglo-Norman grevous
Origin: , from grever, to harm, aggrieve
Origin: , from Latin gravāre, to burden; see grieve.
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