Etymology
A mondegreen of and per se and, ⟨&⟩ being read as “and”. Letters used by themselves were formerly mentioned according to this pattern, as in “O per se O” for the particle O or “I per se I” for the pronoun I. “And per se and” thus meant ⟨&⟩ by itself, as opposed to forms such as &c.
The specific form ampersand is first attested in 1795, originally as a mocking pronunciation spelling, but this name for the symbol is attested since 1777 (as ampuse and), when it is already called common (see quotations).