And indeed the OED has an entry for the phrase merry England which further undermines the professor’s assertion that “The word ‘merry’ means strong or mighty, as in ‘merry old England”.
Originally: England characterized by its pleasant landscape, etc. Later (freq. humorous or ironic): England characterized by the robust cheerfulness of its people, esp. in an imagined past golden age (often identified with Elizabethan times).The adjective originally had the sense of merry adj. 1d, but was subsequently understood as merry adj. 5a.
a1400 (▸a1325) Cursor Mundi (Gött.) 8 (MED), [Me]n ȝernis iestis for to here..Of..[Brut]..[First conqu]erour of meri ingland.
NB merry adj. 1d is defined as Of a place or country: pleasant, agreeable in character. Obs.
merry adj. 5a is defined as Expressive of merriment, characterized by cheerfulness or exuberant gaiety, festive, joyful, jolly.