-a-

From of a (as in half-a-crown) or of (as in jack-a-napes from Middle English Jak of Naples). Some terms have fully incorporated the use, as ragamuffin and jackanape(s); others arise from mistaken assimilation to the form, as all-a-gog and cock-a-leekie.

From earlier a (attested by the 14th c.), as a contraction of and and its various Middle English forms.

From French à (“various prepositions”) and related prepositions in other Romance languages. Also from related misunderstandings, as all-a-mort from French à la morte.

Clipping of rat.