ὗ ψιλόν

• υ ψιλόν (u psilón) — bare form
• ὖ ψιλόν (û psilón) — unaspirated

From ὗ (hû) with ψιλόν (psilón) meaning “plain” or “bare,” used to distinguish this aspirated form of ὗ from the smooth-breathing form of ὖ ψιλόν. Attested in inscriptions and grammarians' works, such as Steph.Gramm. and Myrin., with support from Latin and Coptic sources (e.g., Hypsaeus, "he").

ὗ ψιλόν • (hû psilón) n (indeclinable)

1. (Koine) The aspirated name for the letter υ (upsilon) in the Ancient Greek alphabet, used in rare phonetic contexts to distinguish it from the smooth-breathing form.

• Liddell-Scott-Jones, Greek-English Lexicon (LSJ) – Υ υ, τό, indecl., ὗ is aspirated in AP9.385.20 (Steph.Gramm.), 11.67.1 (Myrin.) cod.Pal.
• M. Triantafyllidis, Dictionary of Standard Modern Greek – In ancient Greek pronunciation, υ was pronounced with rough breathing, though this was later dropped in most forms.