2. Designating the first letter or letters of a word. n. 1. a. The first letter of a proper name. b. initials The first letter of each word of a person's full name considered as a unit: stationery monogrammed with her initials. 2. The first letter of a word. 3. A large, often highly decorated letter set at the beginning of a chapter, verse, or paragraph.
Rhyme was most often used in these songs because people could remember rhyming songs better than non-rhyming ones, so rhyming songs endured while non-rhyming ones were quickly forgotten.
Most likely answer: the song may have been used to circumvent a religious dancing ban. What few people realise is that the usual explanation of this rhyme being about the Black Death may be widely known, but it is also totally unsubstantiated piffle. http://www.snopes.com/language/literary/rosie.htm.