Why is the digamma function written as ψ rather than ϝ or Ϝ?

Question

The gamma function is written with the upper-case letter Γ, but the digamma function is usually written with the lower-case letter ψ. How did this notation come about? Why is it ψ rather than the lower-case digamma glyph ϝ, or better still the upper-case digamma Ϝ?

I can imagine that the upper-case digamma Ϝ was avoided due to its similarity with the Latin F. But even that seems like a weak excuse when χ, ρ, and ν show up pretty frequently, not to mention the potential for confusion between x and ×.

Answer 1

For the sake of marking this as "answered", this question was addressed on the History of Mathematics forum: https://hsm.stackexchange.com/a/6862/4170

this function was used, and denoted “psi”, much before it got a name.