Meaning of ψιλον

The question came up in class about whether the letters of the Greek alphabet had meanings like so many of the Hebrew letters do. It's clear that Omicron and Omega are related 'o' vowels as we can tell when Omicron gets lengthened to Omega in subjunctive forms. The very names of these vowels mean "small 'o'" (o-micron) and "big 'o'" (o-mega).

We also know that epsilon and eta are related in the same manner. So my students wanted to know the meaning of ψιλον found in Epsilon and Upsilon. Although Eta (as the long 'e' vowel) relates to Epsilon in the same way that Omega relates to Omicron, they do not correspond in the meaning of their names the same way that Omega and Omicron do. It is not 'e-micron' and 'e-mega'! So does the 'ta' have a meaning in the vowel Eta?

And what does ψιλον mean? I have seen suggestions of 'plain', 'bare', 'simple', and 'strait'. But that also raises the question of the Upsilon. What was the corresponding non-plain, non-bare, non-simple, or non-strait u vowel earlier in the history of the Greek language? It was the Digamma. Both Upsilon and Digamma derived from the Phoenecian letter Waw. Linguistically, that the /u/ sound would be related to the /w/ sound makes perfect sense.

Knowing a little bit of this Greek language history helps us with Koine Greek. Why doesn't the verb ἀκούω contract? The Upsilon has replaced an earlier Digamma. Why doesn't the verb καλέω lengthen its contract vowel? A Digamma had formerly followed the Epsilon (examples from William Mounce, Basics of Biblical Greek, 1993, pp. 134, 160).

But can anyone clarify the meaning of ψιλον and tell us what its corresponding non-ψιλον vowel means?