The meaning and origin of the word grig are unknown. Samuel Johnson conjectured in his Dictionary that it referred to anything below the natural size. A sense that fits in with the lively version of this idiom is a young or small eel in fresh water. The phrases merry grig and merry Greek - meaning a lively, playful person - were both in use in the mid 16th century. But it is impossible to establish the precise relationship between them or to be certain which may be an alteration of the other.