merry as a grig

old-fashioned Jubilant; especially carefree, lively, and full of fun. Tom is playing outside, merry as a grig. Though he acts as merry as a grig, he is dreadfully unhappy when he is all alone.

merry (or lively) as a grig

full of fun; extravagantly lively.

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.