It was Porson who said he could rhyme on any subject; and being asked to rhyme upon the three Latin gerunds, gave this couplet -
When Dido found Æneas would not come, She mourned in silence, and was Di-do dum(b).
In the old Eton Latin grammar the three gerunds are called —di, -do, —dum. In modern school primers they are -dum, -di, -do.
When Dido saw Æneas needs must go, She wept in silence, and was dum(b) Di-do.
E. C. B.
Dido was queen of Carthage, who fell in love with Æneas, driven by a storm to her shores. After abiding awhile at Carthage, he was compelled by Mercury to leave the hospitable queen. Dido, in grief, burnt herself to death on a funeral pile. (Virgil from Æneid, i. 494 to iii. 650.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894